There are a couple of ideas about how to position the main gear on a tail dragger.

The simple one is to locate the axles directly below the wing leading edge. It's simple and works with your average model airplane. If it's a really simple model, there is no LE sweep and it's easy to see where the wheels should be. If the LE sweeps aft, it's not so easy. Most people would use the Mean Aerodynamic Chord location to work out where to place the axles. The picture below shows where the MAC would be on the swept LE of that airplane. The verticle thin blue line that touches the LE about half-way out shows where that MAC would be on that model.

Aeronautical engineers go beyond simple to figure out where to put the axles. The work a lot of numbers and such to come up with a location based on where the CG of the airplane is in the airplane. They don't go from where the CG cuts the wing, but exactly where it is. It's usually above or below the centerline of the wing chord. The model in the picture is a mid-wing and it's CG is very close to being mid-wing. So the yellow and red lines were drawn assuming the CG is where those two lines cross each other. The engineers also work out where the rubber meats the road instead of where the axels are. They know the rubber should roll on the runway about 15degrees forward, as shown in the picture. Oh wait...... the picture shows a 27degree angle, not a 15 degree one......... hhmmmmmmmmmmmmmm

That model had an annoying habit. When the wheels first hit the runway, the sucker porpoises. The wheels would hit and pitch the nose up and the model would BOUNCE. It's really tough to grease that baby in. Why?

When those tires hit dirt/grass/pavement, the model is coming down. The tires hit ground and the ground starts pushing up. The CG keeps going down. The nose starts going up. The tail comes down twice as fast as the CG does. That model's tail wheel often hits the rudder. It needs to be bent out every so often. It lands like a salmon going to spawn.

Why not move the wheels directly under the CG? Drag generated by the tires rolling along the ground introduces a nose down pitch. A grass runway throws a bit more drag into the equations. Moving the wheels forward a touch balances the drag. 15-20 degrees forward works.

anyway..........