Sometimes it’ll be written on the adjustment knobs, sometimes it won’t be, but either way double check before you leave the house by looking at the optic, it’s manual, or by looking it up on the internet. A lot of outdoor ranges are rural and you won’t always have good reception to look it up on your phone while you’re there, so look it before you leave. Most optics' adjustments will be set for either a quarter or half inch at a hundred yards, but you’ll find others that are set for metric measurements, and others with odd click adjustments, like some ACOGs which have 1/3rd inch adjustments at 100 meters. Knowing the exact adjustments of your optic will save you valuable time and frustration on the range.
Simple enough, if your scope’s click adjustments are 1/2 at 100 yards, start by zeroing the rifle at 100 yards. If you want to eventually zero your rifle at a closer or farther range, then you’ll ultimately have to do a little math, either multiplying or dividing fractions to zero the rifle at that range. But if you plan on zeroing the rifle at 100 yards, and that’s what your scope adjustments are measured for, don’t sight in at 50 and then go to 100, just go straight to 100. It’ll save you time and ammo, and limit the confusion and possible mistakes of multiplying or dividing fractions - easy as that may seem, it can get confusing. But to make sure you hit your target with the first shot at a longer range like 100 yards, you’ll need to do two things. The first is:
Don’t set a puny 12x12 inch piece of paper for your zero target at 100 yards. Go to walmart, the grocery store, or a crafts store and pay $1 for a big piece of poster board, or a big piece of butcher paper, or something at least 3x3 feet. The bigger you go, the more likely you’ll be able to see your first shot, even if it’s way off, and be able to calculate and make the appropriate adjustments for your second shot. The second thing you’ll need to do to make sure you hit a 100 yard target with your first shot is:
An easy and inexpensive way to boresight rifles that let you look straight thru the bore, like bolt guns and ARs, is to simply set the rifle or upper on a secure rest on the range, pull the bolt out, and look through the rifle’s bore until you can see your target downrange, centered in the bore. Without touching or disturbing the rifle, look back and forth between the optic and bore, making adjustments to the scope until the reticle is centered close to the middle of the target, as it appears through the bore. Using a 100 yard target, I was able to hit just a few inches left of center with the first 3 shots while zeroing a Primary Arms 1-6 for the first time. That was luck, but you’ll be surprised how close you can get with this method.
Lastly, while you may be tempted to use any cheap old ammo laying about to zero your rifle, to save time, money, and a headache, use ammo you know your rifle shoots well with. This doesn’t necessarily mean use expensive match grade ammo, and doesn’t discount lower cost ammo, it just means don’t put any old crap that’s untested in your gun, regardless of price, through the rifle to sight it in. The better groups you get while zeroing, the easier it is to make true adjustments to the scope to bring it on target. It’s hard to accurately zero a rifle with ammo that’s all over the place and giving you a 4 inch spread at a 100 yards. If it’s a new rifle, and you don’t know what it likes, then you have to roll the dice. But, if it’s a new optic on an already tested rifle, then use what you know that rifle groups well with.