Why not practice from field positions in field conditions? If you don't know whether you go go from a 1 moa shooter at the bench to a 2 moa shooter off a pack, or a 5 moa shooter off a tree branch, that blanket rule doesn't work. If I know I'm 1.5 moa seated with a tripod for a 5 shot group because I do it regularly, that's way better than assuming I'm double.
I think the two main issues with this approach are:
1) group size is going to be additive, not a multiplier. Mechanical accuracy plus shooter error. lets look at an extreme example to make the point clearly. If you have a legit benchrest gun that shoots in the .1's from bags/rest, you would be silly to expect .2-.3 moa precision standing and resting your rifle on a tree branch. You probably have a 0.1 moa gun plus a 3 moa wobble zone, for a 3.1 moa capability from that position. Conversely, if you have an old rifle whose mechanical precision is 3 moa, on good bags/rest that let you hold .5 moa (3.5 moa total group size), going to a good field position might increase your wobble zone to 1 moa for a total group size of 4 (not 7).
2) This method is just a guess if you don't go practice. Do you know your precision from prone off a pack? Seated off a tripod? Standing off hand? I get what you're saying, and it's WAY better than the "I shoot .5 moa groups off the bench sometimes, so my rifle is a .5 gun and I can shoot deer out to 800 yards." crowd. But there's (IMO) a better way to look at it.
