What I have heard of guys doing is setting up a 1/2" PVC pipe parallel to the ground about 7-8' or whatever high and adjusting where you stand so that it is in your sight line relative to where your line should be, then have a 45 degree angle going up and out at both ends. You may not actually travel right down the line, but in your peripheral vision you see it as a reference point to get the right line or angle. It sounds a little quirky but its really pretty practical. Also I hold my Tx antenna pointed up into my peripheral view as a reference for center, works good as long as you are one who turns their head and not their whole body.