What Fritz and Pops said.
The datum can be anywhere so is meaningless for gear placement
Directly under the leading edge of the wing should be a good starting point(when level)but may have to be changed depending on a buncha other factors. The Legal Eagle should be ok under the leading edge. The Double Eagle was designed with the axle under the LE but later Leonard came out with a change to move it forward 6". It was nose heavy on the ground. OK airborne.
Dave Thurston’s recommendation is right in line with the above: the wheel/ground contact point should be about 13.5 degrees forward or the CG with the aircraft level. However, he also notes that, for aircraft with a high thrust line, the angle should be increased by 4 or 5 degrees to prevent nose over during rough field operations. The increased angle also increases the load on the tailwheel and increases the tendency to ground loop.
Like Joe says, in line with the LE is a good starting point. The longer the LG, the more weight you will have on the tailwheel when the fuselage is tail down. More weigh on the tailwheel does't help with the ground handling. So going to the higher bush tires does make a difference.
The Baby Ace has the wheel axle in line with the LE of the wing and is light on the tailwheel and easy to nose over. The 4 seat Bearhawk has the axle 2" forward of the LE of the wing and is heavy on the tailwheel, the LG is long so not to hit the tailwheel first in those max slow bush landing at a high AOA and is a good handling taildragger. Axle placement perfect.
Cessna 120/140 's are also light on the tailwheel and easy to nose over on the prop. There is an STC for an axle extender of steel that moves the axle forward about 2.5". One time I almost put my daughter's C-140 on its nose after taking the axle extenders off.
On my Falconar F-12 , I used Cessna 152 round LG legs, axles on the LE of the wing. Placement was perfect but the LG was about 2" to short , would hit the tailwheel first on a 3 point landing and slam the main gear on with the first balance and each balance got worse. So put the top of the nose on the end of the runway and fly it on, with a landing with the tailwheel about 1' off the ground. Get the nose any higher and you just made a BIG mistake.
Yep, like Joe says, its a good place to start, but don't be surprised if you need to move it a small amount to fine tune the airframe. What the test flying of a new homebuilt is all about.