That's really helpful information! Thanks.

To be honest, designers often don't know until they have actually flown the model. They can make best guesses based on experience flying prior models and looking at other designs (and making formal calculations) but in the end, until it's flown it's all guesswork. With such a skilled modelers (and flyer) as Dave Platt maybe he was just a much better pilot than I am and could fly the thing balanced at 12cm.

At 10cm back, my model would definitely be a bit nose down.