Hi there. first time poster here.
I have cut my lower legs to match my (equaly) skinny ankles. they too just barely fit around my boots.
I assumed it was a perfect fit. at least that was when I was standing still. During my first proper troop, I noticed (while walking) that the lower leg parts slided upwards. And during walking, they did not slide over my boots automatically (because of the tight fit.)
As a consequence, I have had some nasty plastic cuts behind my knees, where the lower leg and the upper leg came together.
the bottom gap of the lower legs should be as big as possible, while still being aesthetic and in proportion.