Björn
I like your thinking I know for a fact that 250 GP bikes like the Aprilia had rake around 22 degrees to 20 degrees and won world championships.
There are couple thing to keep in mind .
1. Rake is not really that important. Getting the trail adjusted the important thing to get right.
Tony Foale website has a handy calculator. to to help you run the numbers.
Here is a link to it
https://motochassis.com/freeware/ The first program is the one you want.
For wheel diameter use the tire diameter radius. Have fun playing with it.There is article on his website talking about the relation to the angle of the steering axis to rake.
Here is the link
https://motochassis.com/articles/experi ... -geometry/ 2. I would get my forks and wheels I was going to use worked out first if I having a frame build. The reason I say this is, I am adapting a set of later Ducati 996 USD forks to a TT2. The problem in doing this is the steering bearing in a TT2 frame uses an 52 mm OD taper roller bearing and 996 triple clamp uses an 35 mm ID bearing. Guess what an 52 mm OD by 35 mm ID bearing does not exist. If you making a frame the head stock would be made to fit the triple clamp you are using. In my case I would make the head stock 13 mm longer on the bottom and make it so you could installed 35mm ID by 55 mm bearings. What I did is a story for another post.
More later
Marc