Yes, you are correct that TG doesn\'t correct for ground loss in straightaways for the reason Rick lists, it\'s more than less trivial. Also it\'s a good bit more tricky to measure accurately. You cite the quarter pole and finish line which are convenient markers. But horses often don\'t detour at as conveniently measurable markers as we\'d like. So to get an accurate measure of the questions when, where and how much could be quite painstaking and only for a trivial amount of ground loss at that. That\'s not the case on turns for which there\'s a simple formula and more substantial ground loss as a matter of course, i.e one path wide on the turn is equivalent to a length, roughly 10 feet.
As Rick mentions the reason(s) why a horse zigs or zags in the stretch is apt to be much more important as to gauging a horse\'s form. And we do note if a horse bears, drifts, or lugs out (bo) or conversely bears, drifts or lugs in (bi).