Voltage drop is a function of voltage, amperage, wire gauge (cross sectional area), wire material and length. If it seems like a complicated formula, it is.
Instead there are several calculators available. The one I like for our needs is found
here in the left column.
edit:
Please save me a trip to the boat for a simple answer to a simple question. What size are the 2 bolts that mount the coil bracket to the rear of the engine block? 1/4" or 5/16" if I remember.