when you use a negative correction to remove fuel it moves you backward in your maps and in turn advances your timing=bad.

if you want to keep the afc for a gauge, then get a tuneable ecu with crome so you can tune both fuel and timing. with this then you'll be able to remove the fmu and with the right size injectors have a real tuneable system.

if you insist on keeping fmu and afc (modified hack) then it depends on how much correction you do negative as to how much you should retard your base timing at the distributor, but you still don't have anything to control the timing retard under boost. then you need to buy an msd btm and retard .5deg to .75deg per pound.

thing is, if you do this you're spending more $$ that you could do larger inj and tuneable ecu.

what size injectors do you have that it's running so rich at idle? w/ fmu you use stock inj and it should idle nomal, so it sounds like you already have larger inj already.