Most people won't have a 15-20 litre bucket to hand Rob, seeing as 15 litres per minute is considered to be an acceptable flow rate.
But here's a few more methods for calculating the flow rates
If it takes you 35 seconds to fill a 5 litre bucket 5/35 x 60 = 8.57 your flow rate will be 8.57 litres per minute, or, if it takes 1.5 minutes to fill a 10 litre bucket 10/1.5 x 60 = 6.66 litres pm