Weigh bands will give you a rough estimate, just as if you measure someone’s height and their waist you would get a rough approximation as to their weight. A tape measure is just as easy, and calculations are available on line or in previous posts.
However just as important is whether this weight is made up of meat or fat!
As C says, get used to looking at your pigs, look at you pig as it moves, is it moving fat or muscle? Feel down its backbone and see how easily you can feel the bones. Hardly at all - too fat, very easily - too thin. You want to feel the bones without effort but also feel a covering above them.
Ultimately unless you are doing commercial pigs to a weight, then size and amount of fat is up to you. Take them off too early and you pay a fixed abattoir and butchering fee to process less meat. Meat will still be good. Take off too late and you’re likely to have more fat than you need. Neither is a problem, too much fat – just makes me more popular with the dogs who get the leftovers!