Can someone help me figure out the formula for solving Chord DE (see attachment)?

The two formulas I have been trying up till now (with two different results) are...

.50*sin(15+acos((.5-2*.02)/.5) = .3083

or

.50*sin(15+acos((.5-.02)/.5) = .2594

any help will be appreciated. Also, this angle of 15 may need to be changed up/down a bit depending on the situation I encounter, will that be an issue once we find the correct formula?