Is anyone familiar with this method for calculating the relativistic precession of an orbiting body?

When simplifying by considering a circular orbit, the average orbit velocityvis:

v = (2π a)/t,

whereπis pi,ais the semi-major axis, andtis the period of orbit.

The precession rateis:n

,n= 2π [1 - cos(asin(v/c))]

wherecis the speed of light in vacuum.

Converting this to arc seconds of a degree per century, the rate for Mercury is:

" =n(360)(60)(60)(415) = 43"

Similarily for Earth:

" =n(360)(60)(60)(100) = 4"

I have written a paper on this and would like to add attributions to whomever might have discovered this equation before me.

Have you seen this method anywhere? Any help would be greatly appreciated.