RE: wheels - Where to mount???
Well yes and no.
Yes, 15 degrees down forward from the CG is a good starting point on a tail-dragger.
But no, you are showing the balance point on your drawing, not the CG. Unless the wheels are made of lead, the actual CG will be much higher up in the fuselage. This is typical of many model designs, as they show the CG mark anywhere on the plans that is convenient, but it only represents the balance point.
To find the actual CG of a model you would have to hang it from multiple points on the model. The CG is the 3 diamentional point where the vertial lines from the hanging point would converge.
As far as the good starting point for the location of the gear. It also depends upon the surface you will be operating the model from. Grass field models typically need the gear slight farther forward to avoid tipping up on the prop. Whereas, models flown from hard surfaces tend to ground loop easier when the gear is too far forward.