What is the minimum length of the C-loop line to get full depower of a GK sonic or Best waroo type bow kite?
It will vary from kite size and brand.
If you have a kite that your trying to tune, take it out and fly it in moderately powered conditions. Let the bar go up until the back lines just start to loose tension and the kite is parked with out pull.
If you reach the limit of your bar travel and need more, pull on the depower until you get the above result, then measure how much length you used in the depower, then add it to the CL.
As a start point, most SLE kites will need about 30cm min to maybe 40 cm for max depower.
Pick a day when the wind is steady to make the above easier.