Phil, you should have just asked for advice mate!

All you needed to do was have it idling till you got a steady AFR reading out of it on the old FPR (still on old plenum)
Then take it out and note the AFR at 6000rpm @ 2 bar or whatever

Then change to new FPR, and adjust till you got the same steady AFR at idle, and problem solved you have (to within a few psi) the same pressure, which you can then adjust to get the same at 6000@2 bar and you know that its almost identical then

Then change the plenum over, run it up on the same chip and at the point the AFR starts to lean out a touch you know you have just found the point at which it flows better, obviously then richen it up a touch from there.


These things are really straightforward if you have a methodical approach and you would have learnt a lot more about the relative flow of the two plenums too