Flooring Tile - Level Subfloor

I have an old house (100+years I just found out). I'm pulling up the original floor and would like to tile a large area (15x18) with ceramic.

However, two joists in particular are out of level with the others (one is cupped upward, the other lower).

What should I do to prepare this floor? I had planed to go to the original 1" planking, then put 3/4" plywood on top. These joists aren't crazy, but I remember reading that ceramic has to be 100% level to be installed.

I did a search of this forum and couldn't find anything (even though I'm sure it's here).

Flat is the word, not level. What is the size, spacing and span of the joists? You can use 1/2" ply over the planks, then 1/4" cement board, then self leveler over that, since you can feather edge slc's over bcu, but not over ply. Will give you a better bonding surface as well. First that structure info needs to be checked out.

The floor has to be flat, not level, how bad is it when you place a straight edge over it in the problem area?:)

If real bad, you can remove the subfloor down to the joists, sister the lower one with a straight one and shave down the one that's up too far.:)