Adding 1/2 inch extra length to each wing tip
Blackie,
Leave it as is. Your aircraft meets the 1/4 scale rule without modification. By modeling rules, any single dimension can be varied by up to plus or minus 10% and still be acceptable.
I have attended numerous IMAA events and I have NEVER had anyone question the scale or wing span of any of my birds. These events are for fun, not tight control of qualification, or competition.
Have fun with it!
Bedford