Boom end to clew
For me, I pay attention to the distance from boom end to clew. Often depending on the course i'm racing. (formula). The port and starboard sides of my adjustable outhaul system will often become unequal in length. However, after hours and hours of TOW, you start to get a feel for how many cm's of rope need to be pulled through the pulleys to effectively give the required amount of distance between the clew and boom end.
Relax and go fast