Kremi, you even question you own methods, so how is that FACTUAL?
I'm not trying to personally attack you, but I believe DemonSVT's digital scale, corner-weighing method, over your "Guesstomatic 2000" method - anyday.
I've AutoX'd for a few seasons now, and have taken as much as was reasonable off the car (rear seats, sub box, amp, spare and tools), and it wouldn't have exceeded 100lbs.
Think about it and be serious 300lbs?

C'mon, you'd have to had removed your rear doors and hood to hit that!