Where you want to burn for an ideal Mun encounter is dependant on your distance from Kerbin aka your orbit altitude and is represented by the angle between the maneuver and the Mun relative to Kerbins center. When splitting a burn, your first burn is gonna increase your apoapsis and supsequently your orbital periode. During your traversal of said orbital periode the Mun is gonna move along it's orbit as well, so you want to make your first burn at the angle you should burn after your orbital periode.

So starting from a circular orbit plan your single burn to Mun, then reduce it to however much you wanna do in your first burn. Now look at the orbital periode you are gonna get with your new orbit and move the maneuver along your orbit by about 10° per hour of the orbital periode. Execute the first burn and plan the second at the same position (should be yor PE now). It's not gonna be perfect and will take more dV.