Getting the clock to keep time

The clock has been temperamental over the years so I have been loath to correct it. It has been running slow all that time. The pendulum keeps it in time and time period is directly proportional to the square root of the length. In this case it means the position of the mass. The position can be adjusted up or down by altering the screw. It is trial and error. First go made it run fast. When it is correct I shall be able to take a measurement and see if it does obey the formula.