Starting a Speed Density Tune from scratch ( Cobb Accesstuner)

Im building a VW bug with an ej25 that's running an 07 WRX ecu. Needless to say it doesn't fall into the Cobb staged tunes at this point. Im running Speed density since the intake track needed to be custom built and I didn't want to deal with messy MAF readings. Cobb's software gives me the VE table with everything set to 100 and I'm just curious how I should go about constructing a base table to start off with. Is there some science behind this when I don't have a MAF based map to try and use as scale?

Modifications:

Forged pistons

TGV Deletes

38mm external wastegate

very short 3in catless exhaust

custom air intake

Change the table to 50-60 everywhere and you will probably be able to start and run the engine. From there you tune it -- move to a different cell and figure out what it wants.