Ok, since its a .177 if my conversion is correct you should be zeroed at 20m.
This gives you a pretty forgiving zero since you bang on at 20m and in another 10m it should be less than a pellets width low. This all depends on your airgun and will change with the pellets you use. A 100m is a bit optimistic with a .177 but as long as your having fun go for it.
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