Im new to using a circular saw and would like to improve my accuracy, If I have a blade that is 2mm thick do I add this on after the current line that I have marked and then cut down that? Thanks
A circular saw isn't a super precise cutting tool so you'll want to keep that in mind. Lining the notch on the shoe is not going to give you super accurate cuts. Its fine for framing but may not be for other tasks.
To me a cutting guide is the best way to improve your accuracy. They are very cheap and easy to make, and they speed up your work quite a bit.
Here's a link to one of many how-to's: making a cutting guide
If you make your marks on the waste side of the measurement, you can quickly and easily line up the guide to the edge of the mark, and get a pretty darn close to perfect cut quick and easy.
It really dosenot matter, we all may do it slightly different, one answer says make the arrow on the good side, I was taught make the mark on the cut side. What matters is that you cut at the measured point, I use the notch on my saws to get close but actually look at the blade in relation to the cut if it needs to be close. With framing a blade width is not a big deal but you will find once you pick a method, measure 2x and cut once you will get a repeatable length. Make sure to use a Square so your cuts are at a 90 this is more important than a slight length difference in my opinion.
It really comes down to what your error tolerance is, and then personal preference.
For coarse cuts, such as those needed for rough framing, you generally don't need to sweat this small detail.
For more precise cutting, you'll want to consider a number of factors, including (a) the actual width of the cut from the saw blade (its "kerf"), (b) the thickness of your pencil marks, and (c) your ability to cut exactly along the intended line.
You can handle (a) and (b) by choosing a convention based on personal preference. For example, that the resulting piece of material should go up to the pencil mark, but should not include it. Once you have that convention in place, it's a matter of adhering to it when making your marks and when positioning your saw blade.
Handling (c) is a whole topic unto itself, but there's no shortage of products, tricks, and internet videos to help you with that part.
If you're coordinating your work with other people, you'd need to make sure you're all on the same page. I don't know if there's some convention regarding this that's widely assumed throughout the construction industry.
If you need a peice of wood 10cm long from a 20cm peice, you mark your line at 10cm. But your blade will make 2 cuts one at 10cm and one at 9.8cm if measured from the other side. Normally you cut off the waste because the peice you are measuring is what you want. Sometimes you need to add the blade. If you are marking 3 pieces at 10cm off a 40cm peice your first mark is 10cm then 20.2cm and 30.4cm.
You should measure and mark like someone else is cutting. If your board needs to be 47 1/16" your mark should be straight at that exact length.
Someone who is accurately cutting should cut so that you see the mark but no gap.
That brings up another point. This is why people mark with a pencil. I have worked with guys who mark with a sharpie or marker and it is like why even measure? Your line is 1/8" thick. Where do I cut this?
Marking this with a carpenter's pencil, marking it at the accurate location, and cutting it right is not only an art but an assembly procedure for any crew. If you have a 3 or 4 man crew framing - especially during a build - the guy marking will often not be cutting.