You need to use a good scale too, cheap scales will change depending on where you place the weight on the pan. Use something else and move it around on top of the scale to see if you can get accurate readings. This is the one I have, it's awesome. Right now it's $110 on Amazon but well worth it. I use it for my rats to dial in the 10% of the snake weight and use it to weigh all my snakes, works for all but my retics (of course!). I put bigger snakes in tubs, small snakes balled up work fine right on the scale.
https://www.amazon.com/gp/product/B0...?ie=UTF8&psc=1
I use this one for my retics, not too good for smaller snakes like ball pythons but anything over five pounds is perfect, works great for my 30 pound dwarf retic!
https://www.amazon.com/gp/product/B0...?ie=UTF8&psc=1