You and I have a similar rig. They are single-spreader "telephone pole" rigs.
You have upper shrouds and a single set of lower shrouds. Buy or borrow a Loos Gauge. The gauge has notches cut into the side used to size your rigging, this tells you which scale to use on the gauge.
For our rigs, setting the tension for 15% of breaking strength is normal.
The Selden rig tuning guide will get you going:
Selden Tuning Guide
Mine's a little more complicated, as I have forward and aft lower shrouds. Yours should be cake.