Calculated formula in schedule

[laura.johanssonPN37Y]

Hi, 

I would like my Door schedule to calculate a formula for me but I keep getting "Inconsistent units" error. 

I have doors that have fanlights in them. They are set to 0 unless somebody wants a fanlight above door then they just type in a value and the fanlight appears to that height. I have a door height parameter that gets reported in the schedule, and the fanlight (Hi Lite) that gets reported in the Schedule. How do I combine these two parameters to show Total Height=Height+Fanlight, so eg. 2100mm+500mm=2600mm and when there's no fanlight 2100mm+0mm=2100mm.

I keep trying different parameters and they all come back with the error "Inconsistent units"

I've tried typing in: Height+Fanlight, (Height+Fanlight) and other options. 

Is this because my "Height" Parameter is made up of a formula in the family? 

TIA!

[laura.johanssonPN37Y]

I figured it out, I forgot to do the /1mm. 

However now I have a new problem. Some of the doors DO have a fanlight parameter, others don't, like a rollerdoor. The formula works until we get to a rollerdoor or something like that the it's just blank. 

So now I want a formula that is an IF/THEN statement. Something like this:

IF(TOTAL OPENING HEIGHT<0, <Height>,<TOTAL OPENING HEIGHT>)

But this formula doesn't work of course.

I also tried this formula, but it told me "unexpected >) 
IF(<FAN LIGHT>, <(TOTAL OPENING HEIGHT/1mm)>, <(Height/1mm)>)

Any ideas?