Or you can use the static compression (BDC TDC),instead of the dynamic.
BORE X BORE X STROKE X.0031416 DIV BY 4000.
Example for a stock Kx250, mine to be exact.
66.4mm x 66.4mm x72mm x.0031416 /4000 =249cc this is the cv(cylinder volume).. After this, locate the piston at TDC seal the edges of around the piston like MR GORR suggested. Dont forget to use the head gasket when you measure the ccv(combustion chamber volume)put the head on, bolt it down,and start filling it up TO THE BASE OF THE PLUG THREADS .Take note of how many cc's it took. My 00 kx 250 CCV WAS 17.8CC CV 249CC (CV+CCV/CCV ) 249+17.8=266.8/17.8=14.9:1
Now of course the dynamic is going to be lower than this, because you are using the effective stroke volume(exhaust port closing).I have read graham bells book(very good) From what he was saying about the 2 different ratios he prefers the uncorrected(static) compared to the (dynamic) corrected or japanese way.using the dynamic let's say the volume of the cylinder (effective stroke) off the top of my head is 30mm instead of 72mm which gives you the volume somewhere around 120cc 6.1 cr or what ever,I know it is somewhere in the 8.1 range. so the cylinder only has this amount using the dynamic.He was explaining this cannot be accurate,because of the return charge of the pipe replacing the cc's of mixture back into the cylinder that was temporarily lost.Plus the high rpms of the piston will out run the fuel charge before all of it escapes out exhaust. uncorrected way for me
So pick your CR.