My own Cessna is a good example, I know exactly what the max weight per seat and the max fuel is before I have to re do a formal W&B. At those max figures I know I am inside the envelope, anything less and I am inside the envelope. Simples!!!
Yep, I do(did) the same. W&B for my standard configurations. Anything else and I run the numbers.
Same with landing and take-off distances. 400m grass or tarmac is good enough when <2000ft AMSL and < 25C with no wind or slope. Otherwise I run the numbers again. For fuel burn I've measured it for a while. Now I know it is 18.5 l/hr(using 20l/hr to avoid headaches) @2450rpm giving me 85kts.
-Kees