I'd work with what you have. Buying more cases is meaningless, unless by some miracle, you got the same lot number.
Assuming that you have a marked board laid out, and you can view the results of your bell curve.... as the cases are arranged, select whatever group you want; heavy, light, or right in the middle of the range.
I divide my brass into three groups, after discarding the 5% HIGH AND LOW. So, if I start with 200, there will be 20 discards. From that point on, I divide the groups as best I can, using no specific formula. It might be 50- and 50+ and 80 in the center of the range? Or it may be more equal.
I have not determined that any of the three groups are the more accurate, and consider them all to be useful, as long as they are kept separated.
Okay, that's my method, as far as it goes, as to weighing cases. I do other things in my selection and culling process, but even the discards can be useful for some purposes.
So, my question would be, are you sizing your brass before weighing. Are you removing the flash from the flash hole and uniforming the primer pockets before, or after weighing?
Are you weighing before triming and chamfering the necks, or just weighing raw cases? Whatever you do, develop a system that makes sense to you, and stick to it.
Good luck, LB