Gilo,
I'm no expert either but AFAIK this is the only way you can measure spreader rake.
Place a batten on the wires and measure the distance to the mast.
IMHO the spreaders should only be set once according to your combined crew weight, you do not want to adjust it every day basically because its a PITA to adjust. <img src="http://www.catsailor.com/forums/images/graemlins/cool.gif" alt="" />
You should then adjust diamond-wire tension to suit the conditions (depower), and bring a tool along so you can change it on the water.