I generally do not use a foot stool, going with a gitano instead. When I do it is slope up. At my teacher's, he has a stand with an integrated footstool, and it is flat. I can't say I like it much.
I should spend some time analyzing this and my position. We all have our unique body mechanics. For me, I am tall with long arms (I cannot wear a standard shirt, for example). Long arms are good for boxing, bad for weightlifting. I don't know what their impact is on CG!
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