There is a Sticky thread here in this forum on "Reading a degree wheel" where you can learn how,please read thru it.
For your stock intake port I would suggest you lower it 2mm,this is a safe amount to alter most stock intake ports for more duration resulting in better power without much real negative affects. Alternately you can also trim the intake skirt of the piston for the same result. When doing this try to keep the same shape/curve of the bottom edge of the port as stock,it must be slightly rounded,not straight and flat across. Also it is vital to chamfer the edge of the port after you alter it,it must have a bevel to prevent the piston from contacting a sharp edge and to help guide the piston skirt past it,again look at the factory chamfer job on all the top and bottom port edges as a guide. Failure to chamfer the port edges can result in the damage of the piston or rings,in many cases I will make a better chamfer job on the cyl. ports than stock for reliability and engine life. You can use a round file to do this,take your time and use care,you can chip off the Nikasil plating of the bore if you are not gentle.