duration is measured from port opening, all the way through BDC til port closing again... you can assume its normally twice the figure you get from port opening to bdc, unless its a desaxe type cylinder...offset from the crank center line. these particular engines are not.
hardest bit to measure is the transfer timing... its a bit of a guesstimate unless you tear it down, measure conrod length and do some maths.
i usually just remove head, replace two of the nuts with some packers underneath to keep the cylinder in place as if its assembled, and yeah...
the whole things a slight guesstimate anyways, as theres a slight amount of leakage from top of piston to top of rings, unless you got a dykes ring which sits at the top edge of piston. once again, not on these particular engines. no harm in trying to modify a piston to take one if you got a lathe handy... havent ever heard of anyone doing such a thing to a china engine yet.
if youre gunna port, get some dental drills off ebay. dremels/die grinders just wont get in there. you have no control, and its far too easy to slip and take a chunk out of somewhere you dont want to... plus you cant get the angles in the transfers.
do a search for "gordon jennings 2 stroke tuning", the pdf is readily available online. or on here...linked it a few times.