My disclaimer... Sorry, I can't seem to upload things, (I think it is a company IT thing I believe) and I'm not going to try to explain it with words, so I'll just try and give some general advice / helpful tips. I also think this could be solved a few different ways.
So it looks like what you have works fine, but one thing is not working correctly. So just fix that one thing and your golden, not impossible. Just break it down into the basic components you need to control and you have your answer... You will have to figure out how you make it work still, but at least you're on a path. Just keep trying different ways and I think it will work fine. Remember, it doesn't have to be elegant at first, it just has to work as easy as you can make it for the user.
I've handled something similar before, and hopefully something will help you out also. The trick for me was to use an angled reference line and use a full reference circle. Then you can control the location of the center of the circle and the radius easily as that seem to be the 2 things you need to control to fix your current problem. Once I got them to work correctly using formula's I drew detail lines and locked them to the reference lines & reference circle. Flex it to and extreme bigger and smaller to see if you need to lock the line ends, because sometimes there is an applied relationship that you don't have to actually place.
Flexing it will also help you play with it when you placing the reference lines to help you take one more step. Basically think like a mad scientist...
This should work like this... but wait, it worked like this why?
If nothing else remember this:
Just because You don't see a way, doesn't mean one is not there.