Why single-point threading needs multiple passes
Cutting a thread on a lathe with a single-point tool means removing the full thread depth across several progressively deeper passes rather than in one cut, since taking the entire depth at once would overload the tool and tear the thread form instead of cutting it cleanly. Knowing the total depth and how to split it across passes up front makes programming or manually cranking in a thread far more predictable.
How the numbers are calculated
Pitch is simply the inverse of threads per inch. Total thread depth for a standard 60 degree thread form works out to roughly 0.6134 times the pitch, the theoretical depth from crest to root. Infeed per pass then divides that total depth evenly across the number of passes specified, giving a straightforward starting point for how far to feed in on each cut.
Practical tips
Real threading passes are rarely cut in perfectly equal increments in practice; many machinists and canned cycles reduce the infeed amount on later passes since the tool is engaging more material per pass as the thread gets deeper even at constant depth increments. Starting with roughly the first half of the total depth in the first couple of passes and tapering down the infeed toward the final finishing pass produces a cleaner thread and less tool stress than straight equal division.