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...mostly the problems I see here are with people who have frames where the seat post extends some length above the top tube.
Depending on how heavy you are (as above), you need to have a certain amount of post in the section of the seat tube below the top tube.

Usually an inch or two minimum down below the junction of tt/st. You can visualize this by holding your post alongside the st, and make your own mark with a sharpie.