I had a session on the range the other day - not overly successful as very hot, barrels hot, heat haze and thought scope had crapped out as it was shooting a bit too high. Cool of the day its all fine.
But key thing was I wasn't quite how high it was shooting and of course didn't have a tape measure with me.
In the cold light of two days later just sitting at my desk with the target in front of me and its much better than I thought. All rounds into an 1 1/2" group. And have discovered and will patent a very useful measuring stick.
A Cartridge Case.
Now a 243, 7-08 or 308 case is 1 1/2" (give or take a nats testicle) from the base to the shoulder and 2" or 50mm long
So at 100 yds, go to your target, base of cartridge in middle of target, shoulder of case middle of group. Thats a 200 yd zero for most rifles.
For those with longer cartridges
7x57 - shoulder is at 1 3/4" and case length 2 1/4" or 57mm
And the 7x65R (long 30-06 length) is 2" to the shoulder and 75mm long overall (a bit over 2 1/2").
So there in solves the problem.