Lots of people shoot the black disc out thinking they have zeroed - er not quite........
This applies for a target rifle but can be adapted for a scope or service sight:
A weight on a string makes the target vertical set the scale under the string & pin in place- so DO NOT CANT the gun. a spirit level on the rear sight sets it as level, then you can level the foresight to that, so when you wind up to 1000 but for a bit of spin drift you will be in the black, if your foresight is not level with your rearsight or you cant, you will go out one side (right top for a RH shooter).
Dont zero on the black - Zero for 600yds (i.e. aim at the black hit at 600yds) & deliberately 2-4 moa windage out to one side (right) so you dont chop the scale up. All but 1 of our club guns are set at 13moa = 600yds (the other is 23moa!) slip the plates when at 600yds to 13moa or a multiple with 10. (or 3-rear +10-fore on an adjustable foresight) Then move the foresight / wind up to 900/1000 noting the settings & back to 600 then down to 500 / 300. That splits the difference of error so when you go to 1000 you have a good chance of being on the black vs if you start from zero/300 & just wind up cumulative errors can put you 1-2-3 moa out & the target is only ~4moa high / low.
Look at the pattern of shots up & down the card if not vertical then something on your gun is not level.
If vertical finally wind off the preset 2-4 moa & slip the windage to zero.
If you want to zero for 2 ammo types apply 2-4moa wind the other side (left) & repeat but keep the elevation for 600=13 moa for 155grain on a 308win. Just note the different settings.
Then refine those settings by recording your best elevation per ammo per butt per distance from your plot diagrams. (butt because at Bisley they aint level & you can gain lose an moa because of that). If your lucky on a dead calm day 300yds you can refine the windage or again take the best windage setting vs the flag indications & average them out. so e.g. a perfect 3oclock wind indicating at 2 moa flag - if your V centre on 2,5moa your out by ~0.5moa you can take a good bet & refine your windage by 1/4moa at least.
Once set try not to frig with them..............
Draw up a table e.g.
155 grain RG / (GGG) / Foresight Iris setting
& clip into your score book / weather writer (laminate it)
Distance..............Elevation...............Foresight..............Rearsight.......FS
1000.....................36..(34)................35.......................1...............2.8
900.......................27..(26)................25.......................2...............3.0
600 ......................13..(12)................10.......................3...............3.6
500 .....................7..(6) ..................5 .......................2 ..............3.8
300 .....................3.. (2)...................0 .......................3 ..............4.0
Note the rear sight hardly moves so you dont change you head position vs distance. & you close the foresight down as things get further away NB the black for 800/900 & 1000 at Bizzles is the same size so they get smaller as you fall back.
In TR you get 2 sighters the above makes sure the 1st is in the black & the 2nd can be converted to a V, remember VVVVVVVVVV or 5 VVVVVVVVVV beats 45 VVVVVVVVVV so converting sighters gives you an edge in ties.
Now spend the next 4-5-15 years learning to read the wind & winding the sights in predictive mode: Shot on 4, should have been 5 for a V when the flag is in 'that' position. & in 'that other' position 3 so now you have bracket. so now you look, wind on & shoot. Some people take every shot as a 1st sighter by setting their windage by what the flag says vs what the last shot did. If i lose the plot (i do that occasionally) I do same - wind to zero, & reset from the flags or if the wind is mad / variable.
NB the target centres at Sticks are 18moa so if its blowing a howler you can bung on say 5 & aim up wind on the adjacent target & you are shooting on 22 moa wind

but it can work.
People with scopes can just aim off using their graduations to do that in a calibrated way so less messing about twiddling things.