Use one rubber band per side per pound of plane weight. i.e. a 5 pound plane should have 5 bands per side. Now add one crisscrossed each way and you got it! I advise to NOT reuse rudder bands from day to day. Glow fuel, sunlight, and constant stretching quickly cause the elasticity to decay. The old "store them in cornstarch" trick may dry out the oil on them, but does nothing to restore the original strength. Rubber bands are cheap, especially when bought in bulk from Office Max, Office Depot, or Staples. There's nothing special about "radio control" rubber bands. A "#64" rubber band is a rubber band is a rubber band.