It seams that every logger has there own unique technique of measuring and bucking a tree to length. As long as it is an accurate and efficient technique than there is no need to change. There are a couple of measuring tools and techniques that are more common than others.
We will call the first one the “flip the stick method.”
Find yourself a nice and straight stick preferably green and hard like oak or ash. Use a tape measure and cut the stick to either 4 foot or 6 foot long exactly. I like to use 4 foot long sticks because most log lengths are divisible by 4 foot. Mark the stick by cutting a groove in it at 2 foot increments. You will want to cut another groove at 4 or 6 inches from the 2 foot mark so that you can measure the extra length that is required by the mill. Most mills require the logs to be at least 4 inches longer than the exact length to account for any mistakes in cutting.
Start at the butt end of your felled tree and lay the stick on top. Then place your finger at the end of the stick and flip it end over end. Repeat this until you have measured out the length of log you want. Don't forget to add the extra few inches of “fudge factor.” Now you can mark the spot to be cut either with a hatchet or some marking paint.
The other method is to simply use a loggers tape to measure your logs. The loggers tape is just a really long tape measure with some spikes on the end, and a clip for your belt. It is more accurate than the flip the stick method but it does have some problems.
If you want to cut several short logs out of a tree, and they all need to be 4” longer than the exact foot measurement, then you have to keep track of each measurement in your head. By the time you get to the end of a 60 or 70 foot tree you most likely will have made a mistake and your logs will be odd lengths. You could get around that by cutting each log as you measure so you have a new place to measure from, but switching from sawing to measuring and back again can really slow you down. It is much faster to do all your measurements for a batch of logs and then start cutting.