K.I.S.S!!
SCA = 60/(Speed in mi/min), so at 100 kts that's 60/(1⅔), or 180/5, i.e. 360/10 = 36°.
Thus at 100 kt, SCA is 36° and a timing correction of approx 1/4 the time spent correcting a track error should be added to the next time check, if you're happy with the along track velocity being speed x (cos SCA).... If not, just believe it - because it works!
Don't try and make it too complicated though - it's a technique for correcting back to track simply and quickly without too many hard sums. And you can always note down headings ± SCA for each leg before you get airborne!