okay - well, to do that, you will need one of those spring loaded scales with a weight marker that will stay in place on the weight is released. Neil, I am sure you stock those so if you have a picture of one, that would probably be useful. You will need the aid of another person unless you can attach the scale to a fixed point. The idea is to pull to a weight you can actually manage - make sure it is one you can manage and not one that will have you water skiing.
Once you've established the weight you can manage, you can continue to set the drag this way everytime or another option is if you get a terrific little device made by Bouz. This attatches simply onto the line and you can take the weight measurement yourself.