Which 750DP do you have? Adjustable air bleeds?
If yes, go up .003-.006 on the idle air bleeds, outers. Readjust you mixture screws and see how the cruise reacts. It should lean it a little.
Usually when the mains start to activate you'll see a decrease in A/F reading. If it's still getting leaner, you're likely still on the IFR/transfer slot circuit.
