Right I am trying to set up some 2nd Pike 454 Dual air

I’ve put 115psi in the top, this = 40mm sag

I then added 93psi to the bottom. I stopped at that because the forks had dipped by 4mm.

My sag now measures 46mm and forks feel softer.

Is this right?

yes.

Yep. Let all the air out the -ve before setting the +ve, then do the -ve to minimise the travel loss

Yep, the negative chamber is there to overcome stiction in the fork stroke, too much pressure though and they’ll suck the fork down. Good way to check for other Rockshox weirdness though, like when all the damping oil leaks into the lower leg. Emptying the positive chamber and the forks compress complete;y, unless there is damping oil in the lowers and then they don’t.

I let all the air out of both then set the sag first with the +

What I thought was odd was that the theory gies that
= pressure is linear
More – is more small bump compliance
Less – is firmer

If I put 115 in the – to match my + then I would have lost a lot of travel..