HFD - yes, technically you are right. But, bear in mind that the idea is to have a simple system which reduces your off-track error to a value which allows you to recognise your original planned track visually. Showed a student the technique today (pretty calm winds admittedly) - when we turned back on to the original heading we were within a gnat's of track and were then able to refine our track exactly be reference to ground features. We even got to the turning point within 5 sec of the revised ETA!! If you were to carry out a full analysis of errors in the SCA technique at light aircraft speeds, you'd probably find greater sources of error in distance estimates, pitot-static and DI instruments, rudder trim etc than would result from not recalculating (60/GSmiles per minute) on each and every leg. SCA is a KISS technique rather than a precise navigation technique. But it's simple for students and it's certainly good enough for practical applications!!