What is the Best Rifle Scope optical magnification for my .308 rifle?

Rifle is a .308 Remington 700 SPS Tactical -20 inch barrel (out of the box)

I will be shooting from 100 yds to 500 yds.

Trying to get a good performance with less monetary value. Hence, my choice is the SWFA “Super Sniper”.

Seems to be a mediocre yet tolerable scope with good reviews.

Super Sniper:
10×42 30mm Rifle Scope
16×42 30mm Rifle Scope
20×42 30mm Rifle Scope

To bring down the total cost to less than $375 I plan to pair it with Weaver Tactical Mounts.

you shouldnt need more than a normal 3×9x40

http://www.Bushnell.com/general/riflescopes_legend_75-3940m.cfm

my uncle has that on his .30/06 and he says it works fine