Simon,
Thanks
re: your question
"..have you calculated the new area vs the original? Is it 15% bigger?" No, I have not calculated it. That would require math.

The only logic I used was to add chord to dampen pitch and to add span to dampen roll. The rest is TLAR ("That looks about right").
I don't know if it is 15%. It is "just enough".

It still needs to be scale and not look awkward. I can only judge that when I can pin it to the tail end of the fuse and look at it from all sides. That's why I have not sanded it yet. I have built in significant options that can be adjusted with sandpaper.
The personal guideline I use is that when two options present themselves, I lean to the corrective one. Err to right thrust, err to down thrust, err to wash-out. On small planes (less than 36" span) add to span, add to wing area nearer the tips, err to larger rudder with greater throws, err to more power, smaller prop (reduce torque on take-off), nose heavy, more elevator area, less elevator throw, more aileron throw, lighter than usual landing gear wire (to absorb a bounced landing)....... and most importantly, reduce overall weight.
Each adjustment may be small but the totality of these decisions have a huge effect..... in my opinion.... but then, what do I know?

Frank