If unguided then the average declination drift is the PA error per 6 hours. Since it is non-linear, the maximum rate is about 1.5x the average. The average is probably close enough for most purposes.
You'll need to also take into account your pixel size which combined with FL gives you the pixel size in arc-seconds.
You want the drift to result in no more than about 20% elongation for stars to look round.
At some point, as you improve your PA, periodic error is going to become dominant.
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