I've never floor pressed before and I'm wondering whether I should expect to press more or less than my bench. I know the ROM is shorter but you also don't have the leg drive so idk.
I think its fairly situation dependent. I floor press less than I bench, fwiw.
Why not just do your warm-ups and base your training weight around that, just like any other movement you'd ever train?
It is a lot like the decline. First few times you do it it will feel a little weird and you will use less weight than a regular bench. If you all do is floor press you will find it is easier than a regular bench due to the shortened ROM. If you are very thick without super long arms (ie the bar almost touches your chest in the bottom of a floor press) then it will be similar but you will have no arch or leg drive. But start off kind of light and then work up in weight after a few weeks
Try using different grips. My wide-grip floor press is a little short of my 1RM wide grip flat bench, but my medium grip floor press is way lower than my medium grip bench press.