I don't think you can tell the spring rate from the wire diameter used to make the spring. The spring rate has more to do with the differences in the heat treatment process applied during manufacture.
If you check you user manual, It may well tell you what rider weight the stock bike is set up for and what spring is fitted as standard. If your weight is similar to the "stock rider" weight and you can't get enough sag, you probably have a heavier than stock spring fitted. In which case, you want to buy a new spring similar to the one fitted as standard.
Hope this helps.