Jim -

The ones I've seen at fly-ins don't seem to have gaps as large as 1/8 inch.  Maybe half that.  I'd say keep it as small as you can while keeping the gap even (at least on the top, where people see it).

You could run a test to see if deflections under load will cause the gap to close up to the point where the parts touch.  Lift up the wingtip and measure how much the gap closes.  Figure out how much force you lifted with and calculate the bending moment at the location of the gap.  Figure out the maximum bending moment in flight and the deflections should be linear.  (i.e., if the in-flight bending moment is 10 times that of your test, the gap closure will be 10 times more.)

Of course, that will be a pain.  Far better to get one of our already-flying colleagues to tell us how big their gaps are!  I'd bet that none are less than 1/16 inch and none are as big as 1/8 inch.

- Rob Wolf