You bring the material up to the guideline, but not over. The guideline marks are outside the angle that you are cutting.

Another example is that when you measure lumber. You run the ruler up the wood, mark the measurement in pencil, then use a square edge to make the remaining mark.

But when you put you saw to the wood, you do so on the inside of that pencil line. Otherwise your cut will come out too long - the measurement plus the 1/16" pencil line width.

Also, remember that you can turn the cutting mat so that your cut is being made in a comfortable manner to your dominate hand.

I am left handed and almost always use my mat 'upside down' on the table.