Andy Lennon's book gives the NACA results of a bunch of tests on these layouts. ("The Basics of R/C Model Aircraft Design")

The general rule is that (roll control) spoilers should be 10% of the chord, and located at 70% or further back from the leading edge, and about 60% of the semi-span. (WITH ailerons, the span would be split between the two)

There's a page in the book that shows the graphical layout of these and other devices, on a variety of wing shapes... plank, tapered, swept, etc.

I hesitate to publish the graphic on a public forum without asking Andy, but if you like, email me and I will scan it and email it to you. (3 pertinent pages, scanned at a readable size, is 500k)

Spoilers used over the slot lip ("slot lip ailerons") are even more effective (when the slotted flaps are deployed) because the opening spoiler also opens the flap's slot, dumping the flap's lift. (like a Boeing airliner)