Phil,
You can also get a fairly accurate goal BPH by interpolation.
1. Measure the BPH now and jot that down
2. Set the clock hands at the correct time of day.
3. Wait a known period of time, say 12 hours and note the time according to the clock hands. Write that down.
A simple ratio can give you a goal BPH for correct time keeping.
The elapsed time shown by the clock hands divided by the actual elapsed time should equal your initial BPH divided by your goal BPH.
Since the goal BPH would be “X” in an algebraic equation, one needs to solve for “X”
Consider this example:
If your beginning BPH is measured at 1000,
Your hands were set at 12:00 am (The correct time)
You ended your trial time at 12:01 PM (12 hours, one minute)
And if your hands showed 12:12PM at 12:01PM actual time.
{where 721 equals 12 hours and one minute, the actual elapsed time and 732 equals 12 hours and 12 minutes, the elapsed time shown by the clock hands}
732 minutes divided by 721 minutes equals 1000BPM divided by goal BPH or:
732/721=1000/ “X”
Or: 1000 multiplied by 721 divided by 732= “X”
Your goal BPH in this example will be 984.97 BPH, Call it 985.
If you would adjust the rate adjusting mechanism to 985 BPH with your machine, you would be very close to correct time keeping for this example.
The longer the interval between the starting and ending time, the more accurate your results will be. This method takes into account the BPH variation for a short time during any chime/strike warn periods. This is a much more accurate method than published beat rate charts and also calculations using wheel teeth counts.
Dick
Fine adjustment usually takes at least several weeks for any clock, regardless of the type and grade. 
