Ok, you don't really have to measure the supply circuit's current capacity - you only have to confirm that it can deliver full voltage to the Pertronix module. There should be two heavy wires to the module - one is the positive (often red) and may be specified for connection directly to the battery. The other is ground and usually black.

You are describing wires on the coil that are not only reversed for normal operation, but that should have been disconnected from the coil and moved to small wires on the Petronix during installation. The +15 (not 15+) is the marking on the coil positive side and was originally connected to the ignition switch. Coil negative would have had wires to the distributor points (and possibly a separate one for the tach). If ANY of those wires are at the coil with the Petronix system connected, you need to review the installation diagram.

Which specific Pertronix model do you have? I might be able to find wiring colours to be more specific.

Does the igniton coil have any markings on it that state "internal ballast resistor" or " use with external ballast resistor only" ?