How to flatten bathroom floor question

Hi,

My house is about 15 years old. I have a small bathroom (5x8) that I'm installing Snapstone tile flooring in. I'm struggling with a floor leveling issue and am looking for advice. 75% of the floor is flat and level. As a get to the edge of the tub and along a perpendicular wall, the floor isn't flat. I started out using Planipatch to try to level everything. I think I went too crazy with the Planipatch and created more of a problem. I ended up ripping up my underlayment (my floor is 3/4" T&G with 1/4" OSB underlayment) and starting over. What I found under the underlayment was a better idea of why the floor isn't flat.

The section of floor that isn't flat is about a 12" strip that runs the length of the tub, and then 12" on the perpendicular wall (think of a 12" wide "L"). In that 12", the floor travels almost 1/4" off of level. Structurally everything seems fine. Nothing looks bad underneath, no water damage, etc. It looks like this area is where 3 different 3/4" floor boards meet.

What I need to do is find a way to level out the 12" strip (up to 1/4" in some areas). Right now I have the 1/4" OSB out and it needs to be reinstalled. Here are my questions:

1) What's the best way to correct the 12" strip? Should I use self leveling compound? Can this be used just on the 12" wide strip?

2) Am I better off trying to level it under the 1/4" osb or on top of it?

Thanks,

Troy