Columbus makes a spirit seat tube with a 70mm top butt, so they seem to think that is enough. I'm pretty sure they expect you to cut off the bottom. I suspect that 50mm will be enough without issues. I am not sure that DB seat tubes are worth it, the top butt certainly adds a little more uncertainty to extracting a stuck seat post.
One thing that I should have done on my SFL frame that had a stuck seatpost is to cut off the seat post to the length of the top butt. Seat posts nowadays are really long, and there is no need for that. BITD, I think they usually had 50mm minimum insertion, but nobody inserted them much more than that.