Here's a QUICK way to optically centre - But not 100%.
1)Remove scope from gun.
2)Put on LOWEST mag value.
3) Place the objective againsta a mirror that is WELL lit.
4) You should (with 2nd FP anyway) see TWO reticles - the real one and a shadow.
5) Twiddle with knobs until they more or less align - I say more or less as I've never managed a 100% overlap.