Reaching out to the experts here, with what I am sure is either a simple or controversial question.

Recently found myself with a frame with a seatmast that is too short (but a top tube that's just right!) and so I'd like to cut it off and insert a post.

I have everything I'll need, including a reamer to bring the ID from ~30.55 to 30.9, except for an answer to this question:

If I want to slot the front of the seat tube (as opposed to the rear, so I can keep the collar facing back and keep stress off of a possibly thin seatpost), how much seat tube should I leave above the weld at the top tube junction? Or, more coherently, how much seat tube extension above the TT should I have? And as a follow up, how far above the weld should I drill the hole at the end of the slot?

Thanks for any and all insight!

Wishing you all a good time all the time,
Daniel Rietz