Maynard is spot on, but there is a little enhancement I would add. If you need tad more tailheaviness, go behind the spar. If its already a little too tailheavy, go ahead of the spar. Maynard's X'lent suggestion to go "near as possible" to the spar will maintain your current front/rear CG, assuming your CG is on the spar. In a pinch, put it wherever it fits well on the wingtip. You are probably working with such a small weight (<1oz) out there at the wingtip that an inch or two ahead/behind the spar will do very little to front/rear CG. Remember, the fuel weighs ~0.8 oz per fluid ounce of fuel, so a 12 oz tank has 9.6 oz of fuel at ~8" ahead of CG that comes and goes during your flights of glory. I'm guessing that whatever you put on the wingtip, whether ahead, on or behind the spar will do nothing to fore/aft CG compared to using a heavy APC prop vs a wood prop. That's because the props are at ~12" from the CG and have a lot more leverage than the little bit of wingtip weight you'll have to add.

ADVANCED CONCEPT (meaning you can ignore this and still be just fine..): you could shade your wingtip weight position forward/aft of spar to reduce/eliminate any weights previously added for fore/aft CG. For example, if you put the required weight for L/R CG (say its 1 oz for discussion purposes) 2" ahead of CG, you could eliminate 1/4 oz that you currently might have in the nose 8" ahead of CG. Again, its all about leverage, so 1 oz at 2" from CG is equivalent to 1/4 that weight 4x further out from the CG at 8" from CG. Its not magic, its just a bunch of see-saws! Remember how it took two normal kids to see saw against one heavy kid- or you could just have the heavy kid move toward the pivot and have one normal kid....
Hope that helps, sorry if I have offended any normal kids out there!