The scale you use to weigh your car while you build it might not register exactly the same as the official scale at your race’s registration. You might find out you need to add or remove weight at the race in order to meet your weight requirement.
When you build your car, make it about two tenths of an ounce under weight. Then sink a couple of short, wide wood screws into the bottom of the car. Take them back out and insert some lead shotgun pellets or small tungsten beads into the holes and replace the screws.
Now on race day if you need to remove weight, take out some beads or screws. If you need to add weight, put in a few more beads.