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You cannot edit System Families. If you wish to add a Shared Parameter to them, you must add it as a Project Parameter and apply it to those Categories. This means that a formula cannot be used to determine it's value.
Have you tried using the Spot Elevation Tag? It's located on the Annotate tab. I use this to annotation the mounting height of wall-mounted devices because Revit is not able to include this info in the standard Tag. So, I essentially have two separate Tags for the devices in which I need to label the height.
Hope this helps!
Corey D. 
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Thanks for your response. 5 Years ago I would have agreed with most of what you said, however we’ve learned over time that RL’s are a much better and simpler way to install services accurately. As shown in the example below, hydraulics and fire pipes have been utilizing RL’s for as long as I’ve been in the biz (over 25 years now).
If you allow me to give you a better and more detailed explanation, I would have to go back to the time we actually changed to RL’s.
On a certain site, we had a situation where our modellers were called in for a “please explain” on a situation where a drain was not fitting above one of our installed FCU’s. We were looking at the model and everything was clear and coordinated. I’m paraphrasing because I don’t remember the dimensions exactly, however we were saying “But we are in at 2900?” and the Plumbers were saying “But we are in at 36.50??”. In my mind I’m thinking “we aren’t even talking the same language??, and we are both members of the plumbers union??” - We ended up baring the cost for the relocation for the FCU however in later inspection the slab was poured 50mm higher than the model.
Because we were measuring from a physical location and it moved, consequently our services moved up with it. This is where our discovery of the use of RL’s began.
From the next project we started using RL’s as well. This was timely but also out of necessity as the BIM execution plan only mandated levels that were provided by the architect. We also couldn’t have the element referenced from another level (like 0.000) as we suggested. So for example, we couldn’t provide a different level for a wet area set-down, a loading dock or other irregularities in the slab forming; which was very typical for our old CADDuct days. This building is massive and iconic; in particular there were inconsistencies (particularly in the podium levels and carparks) where the only level we had would prove impossible without providing a swathe of sections. Too many to be frank.
As discussed in my previous email, you cannot however get an RL without using a spot elevation which are time consuming compared to tags. To combat this, we decided to tag our elements with the measurement from the level BUT provide a list of RL’s on our Title block to say what that particular sheet was cut at. Here is an example.
The bi-product of this meant that each sheet only had one level, regardless of the amount of steps in the slabs. Because it was RL it didn’t actually matter. The installer would simply add the level RL to the level of the duct to provide the actual RL.
RL’s are provided everywhere on a job sites and are necessary for plumbers, formworkers, builders and multitudes of trades. The below example shows the “+” height relative to the cut level, not the physical slab.
In many cases, like commercial buildings, they are EXACTLY the same (where flat slabs correspond with the FFL) – however it the slab doesn’t, it doesn’t matter.
In this crude example below you have a duct intersecting a ramp, what level is the duct from structure and how do I measure it? From the sides?, From the middle? I would definitely need to cut a section to demonstrate it.
RL – doesn’t matter. As long as you are modelling in 3D and utilizing the coordination tools at your disposal to validate something is coordinated, you don’t have to spend countless hours drawing it. Say you’re running a duct above stadium seating on a slope, how to do you measure it? And from what perspective?
RL’s are provided by the architect, slabs are (well usually..) built and validated to these levels, Elevations work utilizing RL’s.
Here’s and example of structure using the Architects “Level” in basement one which is SSL where as lower ground is not. (I have placed red on the drawing to maintain project anonymity).
I have had discussions with Autodesk about this to provide a solution for both. Show the elevation from level and also provide us with the elevation from Survey point. If we can get the survey point as a parameter, can provide a formulated tag that would be the (survey point + elevation) to give an RL that would greatly help ourselves and other trades who wish to utilize this methodology.
Sounds like you've been in the biz for a while too and I'm sure you've had the unfortunate issue of missing a step in a slab. RL's, doesn't matter.
I've had people come and say "your saying, I have to get another RL measurement every time I drop a duct?" I'm simply saying, "Do it once, then join the flanges together."
Other huge benefits is we used to go round in circles with structural updates on drawings we have already issued because a slab height changed; thus affecting our dimensions to the physical element. RL, doesn't matter (you probably get my drift by now 😄 ) Structure can move and we stay in the same spot because we are measuring from RL and not a physical location. If we need to move because of this change post construction, or more importantly it's built incorrectly, it's a valid reason for a variation; not a cost to us for us to fix.
Long winded explanation but this has proven extremely successful, accurate and drastically reduced modelling and detailing costs. We are also "talking the same language" with builders and other trades.
We've even had some examples of site teams installing duct fabricated and installed without any detailing whatsoever (do to a sudden unplanned removal of a crane). This was done by utilising Dwfx files in BIM360/ACC and querying the metadata directly from the printed dwfx sheet hosted on the platform. We simply gave our site team 1 number. The RL level for what that whole sheet was based from.
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