I suppose you want to calculate the radius of the arc, by knowing the arch chord (AKA door width - left space - right space) and by knowing the ark height.

For the people here to understand, because our conversation looks like an encoded signal, i have attached a file...

So, first to calculate the Semi_Chord. That would be the Chord divided by two:

SC = C/2

Then, the radius can be easily calculated using the following formula:

R = H

^{2} + SC

^{2} / 2H

I asked you to write it down not to forget. But now you can find it here whenever you want