Take a look at http://www.skymasterjet.com/arf339.htm for more information on this model.

You can go up to 40 mm with your flaps.

The typical static ride height for main gear is about 16mm from filling valve center to seal and 14 mm for nose gear from M3 bolt center of fork to seal.

I have my belly speed brake working and it does helps to slow down the plane, but as you have mentioned before, the key is to not approach at a high speed to start with. In mine, typically when I'm turning to base, my turbine is already in idle. This jet keeps so much energy that I find that most time I don't need my turbine keeping power during the approach.

Try you speed brake while flying high and with flaps to see what kind of reaction you get in your plane.