Draw length should always take individual differences into consideration. An optimum draw length will be where the bow is at its most stable.
There are multiple ways to find a good starting point.
Measured
Measure the draw length with a low poundage recurve bow and marked arrow or specifically made draw length measuring device.
The draw length can also be measured standing in the draw position. This requires the person doing the measuring to be experienced, and good form from the archer being measured.
Wingspan
Measure the wingspan of the archer and divide the value by 2.5 and round the result to the lower half an inch.