Wasn't there a discussion a while back about the creation of city-states once the population of the given city exceeded 50% of the state total?
I'm going to have to go with Warren's numbers; as we see too often, 10-15% in each of two or three urban areas can force the rest of the state to do some really stupid things. I'd say mandatory separation at 15%, with an option of voluntary secession at the lesser of 8% or 2 million people.
Look at Texas, for example; D/FW is 6.7 million people, Austin's metro area is 1.7 mil, Houston's 5.9 mil, and San Antonio's 2.1 mil. That's actually a majority of the state when taken together, but only D/FW and Houston metro areas actually have more than 15% of the population each. Essentially, the 13 million people in those two cities can dictate how millions of other Texans must live, and there's nothing that, say, all of West Texas can do about it.
For an illustration of this, look at the Texas State Senate districts; of 31 senators, Houston metro area has 5-7, (five solely in Houston, and another two whose districts are primarily within the metro area) D/FW 7-8, (same issue) and all of West Texas has two if you include El Paso's.
Since Houston is in East Texas and Dallas is just barely out of the same region, their climate, geography and economy are vastly different from what West Texas or even most of Central Texas deals with, and letting them force decisions on the entire state makes about as much sense as allowing Houston to annex Orr MN and apply their code of ordinances there. (even if they did add a 15th council seat so Orr could be represented)
As for California, can we just strip the bottom half of anything useful that isn't welded down (kinda like a Navy ship getting decommissioned), cut all the power lines and overland access points and give it to Mexico, "as a gift"?
Detcord and oxygen lances make "welded down" a rather silly reason for leaving stuff behind...and we should keep the mineral rights.