Update; The Seagull Spitfire has successfully flown. The CG in the manual is definately wrong. An easy way to check the correct CG is to place a finger in each of the front wing mounting holes, if it balances there you are at a good starting point. This places the CG at about 140mm. It will still be nose heavy but will fly well. Thanks to those who helped me figure it out. Happy landings!