You don't need a 10 percent buffer, that's hogwash. Limits mean just that, "the limit". Safety factors are already built in to the limits. On the other hand, you shouldn't exceed the limits either. Having said all that, you look like you got a good handle on the payload number, what it means, and how to figure it.
There should be a yellow payload sticker on the door of your truck, it will say something like "the weight of cargo and occupants should not exceed ________". That is the number that you should be using instead of the "1635".