Used bearings require less preload than new bearings.
Both Timken and SKF used to have some guidelines online, but I have not seen them for a long time. I went to Timken school....
Some lathes use "Precison" Timkens. These need even less preload than normal Timkens because the grind of the rollers is different. It isn't that they are more or less precise than another style of Timken.
A reset used bearing should not produce much heat upon running as there is no break in to happen. Contrary to some beliefs, break in is a real thing. On new bearings installations, heat is normal, and the spindle must be rotated slowly and turned off to cool. A process that can take a lot of time depending on how much rigidity is wanted in the end. A lathe needs more than a car wheel, but less than an index head.
If you sense heat in the bearing area more than the general gearbox oil (10 degrees F) upon first run, you might be OK, but no more than that. Used bearings don't break in unless they have enough damage that they should have been replaced.
For my lathe with used bearings, I'd tighten until I could measure a very, very small amount of drag on the spindle when hand turned. Maybe best to use a string and spring gauge. A 13" inch lathe will obviously need less drag than a 30"
Run it at the speed you want the happy medium of rigidity and cool temp. Faster will get warmer, nothing you can do about that (except use a warm up cycle). It should have almost no heat buildup at the bearing, like nothing (or almost nothing) more than oil temp. If the oil is kept at a minimum (not a flush or stream), it will heat less. Less oil is better. Just a mist or occasional drop is perfect.
I've built custom spindles up to 50hp and 50-5000 rpm. I've done this a few times.