MattR

No, you want to log the parameter "Lambda 1".  Just fix your cal table.
 

you just need to tune your engine, any respetable tuner should do a good job on a NA build like yours. I have tune some BMW engines and if you ask me they normally run from 25-28 of advance at wot with everything tune (good fuel map). The base map is always safe so is normal that a final tune has more timing and normally less fuel aswell. If you are gonna tune the engine I would start with the base map not the one from the rally car. Tune the fuel map and then go to the dyno and tune the timing.

You can do either, often easier to change it while it is running - then you can just bump it up or down until it runs best.

I would calculate it like this:
If your measured lambda was 0.61(9:1AFR).  Lets say your target was typical NA target of 0.87.  So 0.61/0.87 = 0.70 (i.e need to remove 30%).
New master fuel = 20 x 0.70 = 14ms.

If this is not an ITB engine and has a reasonably stable manifold vacuum then you will get a better tune switching your fuel table axis over to MAP or MGP.  Note the existing fuel table numbers wont be much good to you after that as it will take a completely different shape.