The recommended v-belt tension from Gates is all about the belt and how tight it needs to be to not slip on a properly sized drive and driven pulley.
They don't take into account what you are driving and if you have 2 or 3 pulleys in the system. If you are driving water pump
and an alternator with a relatively small pulley, you may expect the recommended tension to not be applicable.
Typically, v-belts will slip some if you have a small diameter alternator pulley on a high amp alternator. Meanwhile, it can also be detrimental to the bearings of the water circulation pump
if you try to tighten a v- belt to the the tension that gives you no slippage at the alternator. There are good reasons that v-belts are being replaced by flat multi-groove belts.