About the Angle

Hi everybody,

I would like to know the value of the Angle A you can see on this picture.

In fact, I build a little physical model and it will be very helpful if I have this value.
Of course, I imagine it depends of constructor, but maybe there are some standards fabrication?

Also, I would know this value particularly on a Cammegh wheel, the Mercury 2 model if possible.
If anyone has this such model of wheel at home, it will help me very much to know this value exactly.

Or maybe, do you have some link about that? the dimension of a wheel, etc…

Thanks a lot in advance!