Hi Don,
This goes back to the articles and columns by Bill Netzeband. For a conventional gear, the place to position the main gear works out to be 15o forward of the CG. This works for any configuration - delta, canard, flying wing, saucer, conventional, swept wing or whatever.
Essentially, this works for full scale aircraft as well.
Now, if you have really smooth surface, you can move the main gear rearward a couple of percentage points and be able to get some really nice smooth landings. But do not move the gear so far back that the nose might tip over. (With the fuselage level to the ground, the axles would be on a line drawn 15o from the CG. If you place the point of contact of the wheels with the ground at 15o, then the axles will be more than 15o to the CG and the model becomes more prone to bounced landings.)
And as has been suggested earlier in this thread, if you are always flying off of grass, then move the gear forward.
(For a tricycle gear, the main gear can be between 10o and 15o behind the CG. Here, just do not place it so far forward of that range that the airplane might just rest on the tail.
Keith