Hello THG
If your Extra 300 is the Extra 300XS these are the suggested throws from the Sig building manual:

AILERONES: Lowrate: 7/8" up 7/8" down
Highrate: 1-1/8" up 1-1/8" down

ELIVATORS: Lowrate: 1" up 1" down
Highrate: 1-1/2" up 1-1/2" down

RUDDER: no rate: 2-1/4" Right 2-1/4" left
listed
Balanceing per manual
27% MAC or 4-3/8" behind the leading edge of the wing. or 2-3/4" behind the leading edge at the wing tip.

Good luck
NEDYOB