Well, not exactly. This may be true at the initial advance, but not after vacuum or mechanical advance is added in. Actually it will be true at just one point along the advance curve. The vac. and mech. advance (or computer!) actually change the relationship of the rotor and cap at firing.
But it is close enough the rest of the time because like DjD said, distributor degrees are half of crank degrees and 42 is really on 21 degrees at the distributor. Or to look at it another way, each terminal is 90 crankshaft degrees apart.
Some aftermarket distributors let you adjust where the rotor is relative to the terminals separately from the timing adjustment.
68 SS Convertible
383 DFI Fuel Injection
on MiniRamII intake,
Trick Flow heads
700R4, 3.31 12bolt Posi
AGR 12:1 power steering