Traditionally the hem of a well fitted gymslip should come to the knee or just above the knee. All of my gymslips come in standard lengths dependent on but/chest size. These lengths are given below.

The 36″ bust comes in a standard length of 40.5″
The 38″ bust comes in a standard length of 41″
The 40″ bust comes in a standard length of 41.5″
The 42″ bust comes in a standard length of 43″
The 44″ bust comes in a standard length of 44″
The 46″ bust comes in a standard length of 45″
The 48″ bust comes in a standard length of 47

If you are of an average height and build then these standard lengths should be fine for you.

If you feel that you need a special length then you will need to measure yourself. To obtain the correct length you should kneel down with your back straight and measure from the top of your shoulder to the floor. Please make your measurement in inches as all alteration options use the imperial system.