The time of flight delta per 100m difference is about the time difference your shell trajectory will be after you adjust your elevation (vertical aim) for the altitude difference. It's purpose on the sheet is just for knowing and calling out the splash time, you can't really do anything else with it. So if your target terrain is 100m above, an adjustment of -12 mils (as if the target as farther than it is, otherwise your shell would impact the terrain short of your target) for the 100m altitude difference at 900m distance, the round will fly 0.1 seconds longer. This is the same for all shells (though they have different ballistic charactersitics meaning different values for different shell types)

For illumination shells, you want to do the same calculation, but then adjust the fuse time to about 1-2 seconds before impact so the flare has time to illuminate and fall slowly. Generally, the automatic fuse timing is very good and you should only do adjustments to it if there is a big difference in altitude to your target.