In general work surfaces should be about elbow high, whether you use them sitting or standing. <G>
So if you stand in front of a cutting table with your arms bent at the elbow to a right angle (about) your hands should just about rest on the table.
Same with sitting at the machine, the bed of the machine should be about level with your hands when you sit with arms bent at 90 degrees.
There are some variations and such, but this is the general norm.
Have fun,
Pati, in Phx