I built a couple of those 20 years ago. A really good flyer.
It is almost identical to the Pirate by Starlink. The Pirate does have an improved fuselage, but needs the center of the wing beefed up a little.
Anyhow, 10 grams of rubber, I think that I use 3 loops for the Pirate that I am now flying. You have roughly 118″ of 1/8″ rubber, so 6 strands gives you about a 19″ center distance. 118″ is approximately 9.8 grams.
With the Potent 30 I did have trouble holding the nose plug in with hooks and rubber band, and if the motor breaks, you will split the fuselage, so don’t put in too many winds.
I once put that model into the massive thermal and it almost went out of sight before the motor stopped. Brings back wonderful thermal memories.
I built a really long fuselage for that set of wings, and flew it in unlimited, and lost it.