You do know there's and entire sub-section here specific to sprockets and chains right?
Any way, here's what my axle adjustment looked like with a 13/48 combo. 112 link chain.
Of course, I'm on an X - not an R....
About as close to centered as you can get.
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2008 WR250X
Gearing: 13t - 48t
Power Commander 5 / PC-V
Airbox Door Removed - Flapper glued - AIS removed
FmF Q4
Bridgestone Battlax BT-003rs