The 18 initial is a starting point, the exact number will depend on the cam specs mainly, to a lesser degree the CR (if higher than stock).
The amount of mechanical advance available depends on the configuration of the advance weights and the "cam" that bears on the weights. The RATE of mechanical advance is controlled by the springs. You want all the mech. timing in by at the most 3000 RPM and less if the engine will allow it w/o detonation. That will depend on the weight, use, rear gears, etc. of the vehicle.
I would use about 34 degrees of total timing. With 18 degrees coming from initial, that leaves 16 degrees that has to come from the mech. advance.
What you will need to do, is clean it up, get the mechanical advance operational and recheck how many degrees it is allowing, and tune it from there.
There are different ways to go about adding or limiting advance from the mechanical advance, but see what you have first. You may not need to actually modify the mech. advance- there's leeway in what the initial timing can be, and that may be all you need to work with to get it dialed in.
Also you will want the vacuum advance to add about an additional 10 degrees when at idle and/or light throttle cruise conditions. You can cross that bridge after determining where you are w/the mechanical advance.