I'm used to oz not cups...but if 2/3cup is about 4 oz, then you've simply carbed the beer completely with priming sugar, so your CO2 tank is just gonna be used to dispense it.
honestly I wouldn't bother priming any kegs, ever. just force carb. then your first couple of pints won't be full of sediment and thus wasted.
Sing with me, "Every pint is sacred, every pint is great. When a pint gets wasted, Bacchus gets irate"
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