As has been said, the math here is slightly mixed up. Using Bill's sketch, we do indeed want to find the dimension of 'a' and as it stands we know angle 'B' (41 degrees being 1/2 82 degrees) and side 'b' at 0.1125". Bill is also correct that we need to use tan(B), which is ~0.869. However, remember from SOH CAH TOA that tan is Opposite/Adjacent sides.Trigonometry - all (most all) hand held calculators have trig functions. Tangent of 41 degrees = .869286. From there basic math. Then touch off the countersink on a suitable shim and plunge the hole for the countersink size needed. Handy for multiple holes for consistency.

Your drawing shows the depth as .0627" which will not result in a .225" dia. countersink.

So therefore tan(B) = b/a

if we multiply both sides by 'a' we get 'a * tan(B) = b'

then divide both sides by tan(B) and we get 'a = b/tan(B)'

which means 'a = 0.1125/0.869 => a = 0.129'.

So touch off your cutter on the surface then plunge 0.129" and you should have the feature your drawing calls for.