I have a dishing tool, but have found that I don't need it as my truing stand gauges both sides of the wheel at once, so (as long as I make sure that's calibrated), the wheel is always dished by the time it's true. But yeah, loosen the side with more space between the axle and the table, and tighten the side with less room between the axle and the table to move the axle to the middle.
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Bikes: 1996 Eddy Merckx Titanium EX, 1989/90 Colnago Super(issimo?) Piu(?), 1990 Concorde Aquila(hit by car while riding), others in build queue "when I get the time"