Hi Iain,
Not sure how far you want to go with this but I do the following......
Using the AFFTA standard for 30' of line make an approximation of the length you will need, this should give you just about a suitable starting point and give you an idea of weight of line you will need for the required length. Measure the entire length of DT you have decided then wind the whole lot on to an empty line spool of known mass, pull off line until you reach the weight you are looking for. By measuring the length of line you have pull off the spool you will know how long the head is for the weight. Be careful how the end of the line is supported as it comes off the spool, if allowed to droop down and pull on the scale platform or is held in such a way that it is reducing pressure on the scale you will get a bum reading.
Cheers
Ben