s10 backspaing help

ok, so im looking at purchasing some wheels but ;what is the recommended backspacing for a 2000 s10 on a 20x8 wheel especially for the rear of the truck.
since its bagged on the stocks i believe they are 15x7 with a 4.5" backspacing but since im going up a 1" more in width how much more backspacing is recommended?? Any help would be great or what others are using on their rides
 
dont know if this helps but my lower bars are mounted to the stock tabs where the leaf springs went so i dont have much space right now between tire and bag its about 1/2" clearence

Typical bs on a 8" wheel for a s10 is 4.5" so if you bolt them up, they will work because they sit in exactly the same place as the rims you have there. Heres a fitment chart: http://www.s10forum.com/new-updated-rim-tire-fitment-for-s-trucks/