It was fine, we basically just kind of used the rule of "If you're on the clear base your in the terrain" even if there's no physical terrain on said base. The bases ironically didn't slide around that much, we had more problems with the actual terrain falling off it.
That is how terrain footprints are meant to work in 11th edition, which I'm actually surprised works so well. It makes things so much easier. Except for vehicles which sucks for T'au since nearly everything I use is also a vehicle. I've thought about getting a playmat that has deployment preprinted. There were ones for 10th edition I know they'll make ones for 11th. Would be nice if it just had a 1"x1" grid so it'd be easy to measure out not just deployment but terrain placement too without too much messing around.