This is what I use to cut firewood to same length. Its called the the perfect stick. Uses a magnet to stick to the bar, I found it works well and fast to mark every 16 inches. I did break the end with the disk by moving to quickly and came off and hit the chain.
I cut the disk off and made another one out of aluminum and screwed it to the end. I think works even better now.
![](https://i.ytimg.com/vi/WgCz6R604nQ/maxresdefault.jpg)