Spring out, fork compressed, hold it vertical.
The reason to measure height instead of volume is simple, if you are just changing fluid, there is residual left in place. If you measure at that point, and pour it all in, you are neither accounting for the quantity left in the fork, nor are you taking into considering the stuff you don't want to wait for in the container. If you measure the amount in the fork, you will know exactly how much is in there.
Today I read an old Motocross Action Mag, they said that they set the oil height to 70mm, later on they wrote "Don`t be shocked by the unusually high oil height" . This is wierd, I found out that this bikes oil level should be 105mm, so the motocross action crew used more oil then ?.
Where did you find out that is hould have 310cc each?.
Maybe I just could put 310cc in each and be happy??.
I got my info from a Team green Magazine that I recently recieved.
Yes, that does mean that the total quantity in each fork should end up in that range.
If in doubt, put in 310, then measure the height. 70mm definitely sounds overfull. 105 is about right, adding more will affect how the fork responds, If memory serves me, it stiffens the compression of the fork. Check the manual for details.
Just dumping it in will most likely not make you happy, definitely measure the height. It is the best way to know.
Even if it does take 500cc per leg, that is half a liter so you'll be good.
The oil level is measured from the top down to the oil, I don't know what the 105mm level is, seems awful low. The specs for a 2001 KX250-L3 show 68mm - 72mm, hence the 70mm spec. I have the same bike and after purging the old oil from the fork I used almost one whole bottle (1 liter) of 5W oil. I would get two bottles of oil. Do just what FruDaddy said, Compress it with out the spring and spring seat fill it to 70mm from the top edge and them wait about 5 to 10 minutes and check the level again, this will allow the air bubbles to escape. Hope this helps.