If it is a long straight log, I'll lay a tape measure on it and mark the wood every whatever inches I want plus a half inch for bar thickness. A fter a few times you get used to the numbers to look for on the tape as adding .25-.5" several times throws off the nice even numbers. Thats if I'm cutting for someone picky. For me I will find or make a reference point on the bar, hold the saw sideways for a second to measure the length, then turn it and make the cut. I also measure with my hands sometimes. one hand is like 8-8.5" middle finger to thumb. That makes 8", 12", 16" and 24" easy enough, and I can over or undershoot as I feel necessary.