Here ya go .... probably the best and most thoroughly comprehensive rig tuning guide on the internet, including a section that yields 'precise' rig tension using only a 'meter stick' although a Loos, etc. tension gage is easier/faster. http://www.riggingandsails.com/pdf/selden-tuning.pdf
Proper rig tension is quite important ... it yields proper sail 'shape', especially the jib or headsail. For most 'cruising' sails, the sailmaker designed and cut those sails 'expecting' that your rig tension was set up to about 12-15% tension.