Clock Repair Archive – –
To calculate the required length for a pendulum you can use this formula: 
where Li is the incorrect length, L is the correct length, E is the error in timekeeping, and R is the length of the test run. Use + if the timekeeping is fast and – if the timekeeping is slow. This is an approximate formula and the results will be as good as your measurments and mathematics. Unfortuantely it is not possible to get an exact answer; but what you will get will be close enough to allow for regulation.
Be sure your error E and length of test run R are in the same units. For example if the clock is 1 minute slow in 3 days you will need to convert the days to minutes or the minutes to fractions of a day before you plug the values into the formula.
where Li is the incorrect length, L is the correct length, E is the error in timekeeping, and R is the length of the test run. Use + if the timekeeping is fast and – if the timekeeping is slow. This is an approximate formula and the results will be as good as your measurments and mathematics. Unfortuantely it is not possible to get an exact answer; but what you will get will be close enough to allow for regulation.
Be sure your error E and length of test run R are in the same units. For example if the clock is 1 minute slow in 3 days you will need to convert the days to minutes or the minutes to fractions of a day before you plug the values into the formula.