A crosscut sled allows a table saw to make accurate, square cuts. The addition of a pair of 1-2-3 blocks will even further increase the possible accuracy.
A 1-2-3 block is a machinist tool used for accurate setup. It is 1" by 2" by 3", either solid or with threaded holes. They are usually sold in pairs.
Woodworkers have know that machinist tools are a good source for highly, well built measurement tools. My squares are metal working as are my straight edges. The 1-2-3 block is no different.
A common 1-2-3 block as an accuracy of +/- 0.0002". (In woodworkers terms, perfect!). The opposing faces are parallel and the side are square. Think of it as a perfect rectangular block 1" by 2" by 3".
They can be had on Ebay for les than $15.00 a pair.
I used them as stop blocks on a crosscut sled. If you do not know what a crosscut is or how to build one, there are several Instructables on the subject.
Step 1: Stop Block Measured From the Blade
For this example I want to crosscut a piece 7-3/8' long. I use a tape measure to place the block at the desired position. Being square it cuts the same length regardless of the thickness of the wood.
Note that I put a thin piece of plywood the surface of the sled. There is zero clearance from the blade to the kerf. Makes it easier to measure.
Step 2: Stop Block Measured From the Fence
I have a very accurate fence that will hold a tolerance of +/- 1/64th. My crosscut sled allows the fence to get within 20" of the blade. To make a 7-3/8" cut, set the fence to 20-3/8" and used the blocks to move in 13". This is done by clamping one block in place and moving the other. 3, 6, 9, 11, and 13, the block is now 13" away from the fence and 7-3/8" away from the blade.
Step 3: More Accurate Resolution
After taking a test cut, I found that the piece was 8 thousandths too long. The stop needs to be moved in by 0.008". To do this you will need a feeler gauge. Place the right sized feeler on the stop face and clamp the spare block next to it (the one with the crooked clamp). Unclamp the stop block, remove the feeler gauge, and move the stop block next to spare block. Clamp the stop block in place and remove the spare block. The stop has now been moved in 8 thousandths .
The same technique can be used to move the stop in the other direction.