floor slopes into room, should I use self leveling cement.

Just put in new subfloor in my bathroom ( 142 year old victorian house) and it is sloping
into the room, I need to level it somehow as it looks like crap. I suggested to my DH
we use self leveling cement. He is not keen on the idea says it may spread to far
and damage the bathtub ( already installed). I said that would depend on how thick
the SL cement is. Also my subfloor is 1/2 inch bluboard over 1 1/2" pine t&g boards, is
it okay to lay SL cement over bluboard plywood. I am using Henry self leveling underlayment cement( it requires the use of its own primer).