The reason it is called INVERSE fraction is because you do just that, but you base it on DTG NOT distance gone ie. at 1/4 way along track you use 3/4 inverted x track error thus 5 x4/3= 7deg. (or near enough) and at 3/4 dist gone it then is 5x4/1=20deg.
another way of doing it is to use the fractions as you have been doing, but multiply by CLOSING ANGLE instead of track error.
