Hi @Jersey1975,
The first thing I'd suggest is to be really sure of your measurements beforehand. You can never trust that walls are square, so you should always measure with the assumption that they aren't.
Use a spirit level to mark a perfectly level pencil line on all three walls of the opening. You can then measure all three sides of the opening along this level line and then measure the distance between the pencil marks at where the front of the shelf will sit.
When measuring sheet products before cutting, I'd always draw the shape on a scrap of paper and mark the measurements on each side so I have a reference I can use to transfer the marks onto the material I am cutting. That way, I don't get confused about what measurement relates to which side.
When you are marking your shelf for cutting, use a sharp pencil and a straight edge to make sure the line is perfectly where you want it and remember which side of the line is being removed. This is because you will always want to cut up to the line, not through the line. Your blade is 2-3mm wide, so if your mark is perfectly on the point that you've measured, if your saw blade runs directly through the centre of the line you will be half a blade's width off which is between 1-1.5mm.
Set your blade depth so it will only just pass through the material. This is the best way to ensure you are not overloading the saw which can cause it to go offline.
You can then use some clamps with your spirit level to set a guide to cut along. Check out what they've done in How To Cut Laminate Benchtop for an example of this in action.
Finally, remember to take it slow, there's no reason to rush.
Let me know if you have any further questions and if you'd like some more assistance, please don't hesitate to reach out.
Jacob
So my best rule of thumb would be to get it cut to a few cms over the length you need and when you get it home do the final cut that is accurate.
