Sounds like you want to have most of your stokers increase their preferred cadence by a significant (or at least noticeable) amount. In that case I'd go for at least the 165 to 172.5 differential between the two cranks. Matching the crank length based on height seems much less important to me - my bikes have a range of crank lengths, 165mm (tandem), 170mm (touring and folder), and 175mm (road) and I feel comfortable pedaling all of them. But I do naturally choose a slightly higher cadence on the tandem with its shorter cranks than on the road bike.