Here’s my thought: if you can’t rely on using standard passenger weights and proper seating guidelines then perhaps you are cutting the weight and a balance calculations just a bit too close…..
Nonsense.
For one thing, the only reason that there is a “standard passenger weight” is that they weigh people occasionally to find out what the “standard” weight is.
More importantly, though, are two things. 1 - the reason that you have standards is so that airlines don’t have to make their own determinations about what’s safe and what isn’t, and 2 - weight and balance doesn’t work that way. If your weights are significantly wrong, the balance of the plane is going to be off no matter how much fuel you load.