If you are just starting out with a kit which has a blown or fibreglass hull, simply mark where the water line is on the inside of the hull, get a spirit level to make sure everything is level and fill with water to the water line marks. The number of litres of water is equal to kilograms and ignoring the actual weight of the hull itself will give you a pretty good idea of how much ballast (or goodies) you can have/or will need.
Once its finished and on the water you will then only need to finally tweek the weight to ensure the trim is correct.
NEVER EVER permanently secure ballast weight or big Lead Acid batteries that cannot later be moved a bit - or you could have a boat that is stern or bow heavy and cannot be trimmed properly, when all the internals are fitted.