Night instrument going eastbound and night instrument going westbound. North or south cardinal headings should be rounded up to the closest tenth of a minute, not to exceed the closest line of longitude; of course taking into the proximity to the departure airport point of latitude.

In all seriousness, I would take the ratio to day to night total time and use that same ratio to instrument time for day and night. That’s my opinion, nobody tracks that stuff unless you have kept a highly accurate electronic logbook since day 1. Ridiculous.