I'd say take gravity of each wort, and use the average. I never took pre-boil gravity until I got my refractometer, which was the only time I did split boils like you're doing.
I also did half my runnings into each pot to keep them fairly equal (for hop utilization, etc).
Primary: English Mild
On tap: Pale Ale, Lancelot's Wheat, English Brown Ale, Steam Beer, HoovNuts IPA
Bottled: MOAM, Braggot, Raspberry Melomel, Merlot, Apfelwein, Pyment, Sweet mead, Cabernet
Gal in 2009: 27, Gal in 2010: 34, Gal in 2011: 13, Gal in 2012: 10