Back in the old days, when I had time to tinker, I would rig up a good solid way to prop up my rifle on the outdoor picnic table or something. Then find a distant object, remove the bolt from the rifle (naturally only works on bolt actions) keep working with the rifle till you get some real distant object centered when you look down the bore. I have use the water tower 4 miles away, or a distant grain bin. Get it centered in the bore, best you can, then without moving the rifle, move the scope till the crosshairs are centered on the object. Takes some "tinkering" but it kinda gets you in the ballpark.