bankroll vs limit question

[Cincy Peach]

It seems to be well accepted that 300BB is a good bankroll to withstand swings. Recently I have began to proportion my tables to reflect this ratio: in other words -

when my BR is $300, I am playing four tables of .50/1.00. When I hit $375, I am adding one $1/$2 table to the mix, at $450 I would be playing two 1/2's and two .5/1's, and so on.

Is this a mathmatically valid approach? or should I be waiting until BR=600 to go to 1/2?