You need to measure the diameter of the wheel attached to the encoder and calculate the ratio with the diameter of the table.
The OP already stated that he cannot mount the encoder centered to the table so your suggestion won't work. If the wheel attached to the encoder is 3" in diam and the table is 12" in diam (for example , round numbers), then you don't have to calculate circumference because you automatically have a 4**:**1 ratio.