This is very helpful information. What is the min / max saddle height on your 56 size frame?
Minimum distance from BB to top of saddle = 70 cm (67 cm from BB to center of saddle rails) - but you could cut the seat post to get even lower.
Maximum distance depends on the minimum amount of seatpost inside the frame you want, of course. But the distance from BB to top of the seat tube is 49 cm and the length of seat post + saddle is 39 cm = 88 cm. I would say that you probably want at least 7 cm within the seat tube = 81 cm as maximum saddle height from BB to top of saddle (or 78 cm to center part of where the seat clamp grabs the rails).
The reason why the minimum saddle height is not lower on the 54 cm is probably due to an identical placement of the rear wheel cut out in the seat tube (which is the limiting factor of how deep the seatpost can go)