The speeds have to match exactly. True speed and Calibrated. I try to think of it as if it's 1 mph off, it'll be 10 feet off...makes me calibrate better.
Correct in that unlike prop bombers calibrated speed and alt need to exactly match E6B reported speed and alt.
Dropping a bomb from 10,000 feet with a terminal velocity of 800'/sec (taken from a NASA bomb trajectory report as a real world example) = about 15.5 seconds to reach the ground (approximately) accounting for vertical acceleration (55'/sec/sec) and misses the target by about 23 feet with a 1 mph speed calibration error.