In alt printing the minimum exposure for black is enough exposure through film's base to match the black of the print where it has been exposed without the negative blocking any light from hitting the paper. This is seen in the rebate -- unexposed portion of negative, such as around (through) sprocket holes or the edge of LF negatives), or the un-printed area of an inkjet negative.
Examining a simple test strip tells one this. Basically, if one does not get as black a black thru the rebate as one does without the rebate, the print will have none of deepest black possible with the material. Then one decides where to go from there.