" how do I know how to return to zero or where it's at "
This is actually very simple.... others may have a slightly different method, but it will look something like this. Since you want 100 (I will assume your scope has 5.0 MIL per complete revolution of the knob)....
1. Bore sight / get on paper at 100 yards.
2. Shoot several rounds adjusting you scope until you are zeroed at this range.
3. Remove your elevation cap carefully to not add / remove a click.
4. Add the provided shims under the knob (the gap does not have to be packed full, but should be close to it)
5. Replace the cap being careful to get the 0.0 lined up with the line.
In use, whenever you want a new number, you spin all the way down until you hit the shims. DO THIS EVERY TIME (especially if you are switching between 250 and 1000 yds as you may need several full rotations and can easily get 'lost')
So after going all the way to the shims, this is ~0.0 (not 5.0 or 10.0). Mine actually spins down to about 0.3 MIL less than 0, but that is totally your choice. Just put it to the 0.0 that will be visable. If you then want 6.9MIL you go 1 entire revolution to make 5.0 (will read 0.0 again) and then continue to turn to the 1.9 line.
Basically the same as above, except for the shims. You will need to be able to adjust left and right, so you can't block it. Just a line the 'no wind 100 yard zero' to 0.0 and make your adjustments from there. NORMALLY you will not need to go more than 1 full rotation in windage.... but it depends on your love of hurricanes and your scopes adjustability.