On MY rifle zero'd at 200, I would be 2" high at 100 (no change to POA) or 7" low at 300. Your best bet would be maintaining a 200 yd POA/POI zero, and knowing the required hold to your maximum engagement range (300). This will vary from one type of ammo to another based on bullet weights, muzzle velocity (which is effected by barrel length, twist rate, and even individual characteristics of similar rifles) as well as the QC lot-to-lot and even round-to-round of a particular round.
Another tip is to make a range card- just a simple sketch of the area you are watching. identify recognizable items in this sector, like a specific rock, recognizable tree, contrasting area of vegetation or dirt, etc., then lase these items with a laser range finder, and annotate them on your card. Do this as soon as you get in position, and store the card on a small clipboard (so it won't blow away) where you can refer to it at a glance. When a target presents itself, consult the card to assist you in determining the approximate range to the target and engage accordingly. I like to take a 5x8 card, laminate it, and fill in the info with a grease pencil. It will hold up in weather, and you can just wipe it off when you are finished for re-use.