Do as Lwerner suggests, specially if you have a VPI with outer ring clamp and spindle clamp - they both add a lot of weight.
I would like to add to this discussion by pointing out that the speed may drift after the TT is switched on and has allowed to warm up and settle in. Let the TT run for about 15 min before adjustments are made.
You can test all these techniques by setting the speed as you have done and with no warm-up. Then, add the record, weights, stylus in groove; and let it warm up. Now check the speed again. Surprise!
I would like to add to this discussion by pointing out that the speed may drift after the TT is switched on and has allowed to warm up and settle in. Let the TT run for about 15 min before adjustments are made.
You can test all these techniques by setting the speed as you have done and with no warm-up. Then, add the record, weights, stylus in groove; and let it warm up. Now check the speed again. Surprise!