Doing a full bathroom remodel. I've ripped out about half the subfloor to replace rotted plywood. I'm thinking of ripping the rest out now because the floor is so out of level.

From the outside wall to the interior wall the joists are almost an inch out of level over 5'. The house is obviously sloping inwards.

My plan was to use 2x4s and cut them to correct the slop and glue and screw them to the existing joists to get them back to level running the length of the joist. My real question is how do I make sure that the perpendicular is still level (or just close to level doesn't have to be perfect) so when I put the new subfloor down i don't have heaves and valleys.

Just get everything closer to level so its not so drastically out of level and then use a self leveling compound?