I have a HD tensioner and set my belt length based on keeping my tensioner in the middle of its travel range. One advantage my setup has is that I have an adjustable idler that I can move and compensate for belt sizes and pulley changes within reason. My belt does not stretch a whole lot as its a 8 rib HD Belt.

 

I like the green hd belts from gates

 

I ended up stepping up to an 8 groove system and placing another tensioner near the snout to increase belt wrap on the supercharger pulley. It worked great and I got no belt slip. I will actually be selling the setup real soon. 2.6 supercharger pulley, 3 extra wide idlers, snout support, alt pulley, pwr steering pulley, 10% OD crank pulley, and the very low mile green gates belt. I just need to get some stuff together and post it up.

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does that rub when it gets hot?

 

No, there is actually more clearance than what it looks like. When the engine turns centrifugal force actually pulls it farther away from the idler.

 

Old method

Find the longest distance between pulleys. You should not be able to twist the belt more than 45* by hand. If you cant twist it at all, its too tight. If you can twist it past 45*, its too loose

 

I reached the limit of the 6-rib belt at 14psi with a 3.25" pulley on my KB 2.8L even with relocated and larger idlers for ~290* of belt wrap. Boost graph looked like a staircase. Boost would start at 14 and drop to 9 by the shift point then jump back to 14 then down to 9 for every gear. Went to a 3.5" 8-rib with overdrive crank and HD tensioner and now see a steady 15# across the rpm range. KB recommends an 8-rib setup on their units for anything over 15psi which I found to be accurate.
I think belt routing and hardware setup are large factors in how well a supercharger will work without slip.