A good ?, I've thought about that too. The spin I now use on my N6.0, is a stock I-20 spin. It only came with one set, kinda high up the luff, and as Mike said, about a foot back from the luff, actually, I think the stock I-20 spin is about 18" from the luff if memery serves me correctly.
However, just as a main or jib, depending on your rig setup, different parts of the spin may be acting very differently, and a leeward telltale in the wrong place may not give you anywhere near optimum 'lift' generation. It's all about 'lift', isn't it?