Your making life to difficult.
Just put a straight edge on the ends of the joists making sure it is level, then pack up the joists intermediate joists as necessary, forget the centre of the joists , job done.
It is not engineering!
that's what i'm doing ,it's what I tried to say in my previous post , I don't know what to use for shim though, I have the small plastic shims but i'm not sure if they're good enough for this purpose, maybe I should buy some 3mm ply and use that ?