The oil volume is just a starting point. The main thing with these forks is the oil level measurement. It can mean you take out oil or put more in to get the right level as per what is specified. What you are doing in pumping the forks before measuring is correct, as it will get rid of air and make the measurement more accurate. I am not familiar with the ST13 specs for the forks, but if you follow the factory specification for the fork oil level it should be ok.
This assumes you have the OEM springs, as if you put in things like the progressive springs the oil level needs to be adjusted, but this is explained in the instructions that come with the springs.