it takes a lot of rounds and research to come up with the approximations necessary to build something that runs right.. if you start at adjusting the port location to the seemingly-correct 60:40 ratio, you still have to adjust for port size and buffer weight. potentially expensive and time-consuming to work out, but definitely doable.

a carbine-length 12.5, for instance, is already exactly at the 60:40 ratio... that should, following the 60:40 theory, already be the perfect gas length for that weapon.