I don't know an exact number, but I can beat the topic into submission for you!
First, find the ideal high speed location using the line rake calculator on the NCLRA web site. For the purposes of a typical profile carrier model, the best high speed leadout position is about 7/8" behind the CG. To keep the plane tracking straight on takeoff, which gives the best high speed, the high speed line guide position should be as far toward the tip as practical.
For low speed, the rule book says the lines must emerge from the model no farther aft than the trailing edge of the wing root. But it doesn't say where along the wingspan the line guide has to be. It should be obvious that the closer to the fuselage the line guide is, the more sweep you get. When you have exceeded the magic combination of too much tip weight, too close a line guide, and/or too far aft a line position, the airplane gets "wobbly" and hinges badly on the end of the lines. At that point in flight, extend your arm or step toward the plane to eliminate some line tension which will reduce the "recoil" as the plane wobbles side to side. To fix it, change whichever variable is easiest to change. You probably won't rip out the slider and move it farther towards the wingtip, but maybe you can limit the slider's rearward travel, or remove a bit of tip weight. Some people seem to get away with a slider half way between the fuselage and wingtip, but to me that is too close. 2/3 of the way out to the tip is probably better.
Clearly the ideal high speed and low speed positions present somewhat of a contradiction. You can 1)Pick a compromise position for your slider, perhaps 3/4 of the way to the wingtip, 2)Install the slider at an angle, so it is farther out to the tip in front, and closer to the fuselage in the rear, or 3) have separate devices to hold the lines in the high speed and low speed positions. I have used all three arrangements. To my knowledge, local modeler Jim Kirby was the first to suggest the angled slider, which was featured on my profile MO-1 plans. Plenty of people use this design. My last two Zeros have used the compromise position slider, and the one under construction will use a separate latch for the high speed line position, and a more inward slider for low speed.
The structure and design of the airplane also add into the equation. A built up wing with internal controls dictates a slider at the tip with limited rearward travel. (Yeah, I know, you could use some dihedral and come out the bottom at some point). Sliders on top of the wing typically necessitate a different design then sliders under the wing. When in doubt, copy something else that has proven to work!