How much injector trim to compensate for non-ideal distribution in forced setup?

[SPatrickB]

I now have my twin screw supercharged V8 running quite well when cruising (thanks to VE Analyzer and a now working MAP signal) but I am seeing richer back cylinders and leaner front due to the radial output of the supercharger pushing the air forward.
I can see the back plugs are blackening/fouling somewhat so I need to trim those cylinders back at least but I don't know how much.
Of course, ideally I would use EGT but I don't have this setup yet.

So, could someone recommend a trim percentage to get me started?
...I'm unsure if lambda scale is linear but if it is then perhaps I need 5% - 10%?

I don't want to go too lean and hurt something, but then I have spent a few hundred dollars on plugs already!

[gunni]

If you take away 3-4% or so as a start you should be good.