The only rule of thumb that we've come up with when testing spacers is... There really isn't a rule of thumb.
We've tested hundreds of different combinations and are usually surprised every time.
Example: We spent two days last week testing different carb /fuel/ spacers combination on a GM 604 racing crate 350 that we had just freshened.
Using gas, we made the best power with a Wilson 1" 4 hole tapered spacer, 80541-1 650 holley and 2 gal 108 Sunoco mixed with 3 Gal. 93 pump gas. This combination picked up around 5 HP & 5 FT.lb Tq. Keep in mind this engine is only rated at 400 Hp & 400 FT.Lb Tq.
We switched over to E85 and a custom built Quick-Fuel carb, and made more HP & TQ than with the gas but made more HP & Torque on the E85 without a spacer than we did with the spacer. exactly the opposite of what we found using gas.
So as Forrest said " It's sorta like a box of chocolate's ......You never know what you're gonna get."
Different engine/ head/ cam combinations respond different.