For a typical engine in front... large main wing, small tailplane in back design.... The best place to start is 25% if you have no other guidance. Starting back at 30% to 35% without knowing how a particlular design responds to the rearward CG is dangerous.

Better... read the plans and put the CG where the plans say it belongs. its a SAFE starting point. (you can almost always move it back some as you get used to the airplane.)

Never go behind 37% unless the design REALLY has a lifting tail. (then you'll be back around 60%.... but its a whole new ballgame in how to trim the model.)