Keeping constraints on elliptical curves?

[j.r.hodgson]

So I have three points on a sketch which are the results of parameters (they actually set lines in another sketch and I use intersects on this one, but I think that's probably not important with regards to my issue).
I use them to draw an ellipse, but because I only want to use half of that ellipse in my extrusion I then do a split of the ellipse and set some of the resulting elliptical curves to construction lines... and this point I get a warning that constraints have been removed, and the lines go from black to blue).
The attached image illustrates the situation. The dimensions shown are all directly or indirectly resulting from parameter valued.
Now, if I change the parameters, at first glance it looked like everything changes accordingly, but if I look more closely I see that this isn't the case.
Now this makes sense to me, constraints have been removed, but I also don't see any way of setting constraints on an elliptical curve, and creating the same curve with other tools is not obvious to me. Is there a way to achieve what I am trying to do?

[davebYYPCU]

Not sure of the request, 

it appears that the ellipse can be made coincident to the lower projected point.

Changing the 17 height dimension, would lift or lower the centre line.

Might help…..

[j.r.hodgson]

I don't seem to be able to replicate the problem I was having before, perhaps because I was changing something else that I haven't realized. In this particular instance, changing parameters seems to be changing the elliptical curves to match the desired overall ellipse, which is what I wanted, but I think the issue I ran into before may have been due to elements not being constrained.

So to simplify the question, when I draw the ellipse through those reference points, it is fully constrained.
But as soon as I split it, it is no longer fully constrained.
What is the constraint that is lost, and how do I get back to a fully constrained drawing (short of fixing things in place)?

[davebYYPCU]

Some constraints are lost - is information, not an error.

To find what to do next, click drag any blue item on screen, watch what happens.  Usually get an idea of what to add, to prevent that movement.

I can tell from the pic, that the small axis line is removed, so Fusion longer has the height of the ellipse under control.

Add a coincident to the projected point.  Will do the same job as the old axis line.

Changing Parameters on a sketch that is not fully constrained, is a nightmare waiting to happen. 

Some nightmares are tolerable.

Might help....

[j.r.hodgson]

Thanks, that helped to a degree, unfortunately as it isn't clear what factors define an elliptical curve (as opposed to an entire ellipse) or even if they constrainable in Fusion I had to find a workaround.