Radius generating using single point cutter

G'Day
I have used this formula to generate large radii over small areas for many years but have not been able to "find" any evidence for it anywhere else.
Radius of cutter divided by radius required = sine of the angle
I use a single point "fly cutter" on a turret mill (so I can tilt the head over about 10deg) with 7" radius cutter to generate 40" radius (for ariel mount pads to go on the outside of aircraft fuselage (Dassault Falcon 20))

How can I find out the amount of radius that remains accurate if any ?
Does this formula have a name?

Cheers
Andy