I have a house built in the early 40's so there is a 1"X6" plank sub floor.
In order to strenghten it up, I glued and screwed 3/4" tongue and groove
plywood over it after securing the planking with screws. The floor is
real sturdy now but, unlevel.
I plan on installing a natural stone floor over it after a layer of 1/2
cement board. The question I have is, is it better to level the floor
with leveller before or after the installation of the cement board?
Regards,
DA

do the cement board first if the lack of level is minor.

if its major level best you can, add cement board, then level again.

you have created a trip hazard with the 3/4 plywood plus cement board..

why didnt you remove the plank subfloor then put down plywood? i still
believe this is a good idea.

how will you transition such a large grade change?