The cardboard is only used to get the original port shape and as a profile checking tool while cutting. I hold the cardboard by hand to the cylinder wall and trace the port with an ink pen. The cardboard is then removed and used as a reference to the original port outline.

With a degree wheel and feeler gages determine how many degrees the duration is to be changed. In a current case 1mm represents 12 degrees duration or 6 degrees difference on the degree wheel with and without feeler gage.

Measure the port height. Something round will usually just fit into the port. In this case a 25/64” drill. Find something else round that is drill size (25/64”) plus 1mm. In this case a 27/64” drill will be the first go gage while cutting.

Cut the port with Dremel tool until the first go gage fits but occasionally check the profile with the cardboard template. Repeat increasing the size of the go gage until finished.

You originally guessed at the amount to increase the duration. Accuracy is subjective but I like for the end result to be accurate within two degrees of target.

Bill