After adding some weight, I would like to know the approximate change in swingweight. I did a search for a few swingweight formulas, but I'm a bit confused. For example, hen inputting the distance the weight is from the buttcap, is it th center of the mass that you use? Also, I read that you take the distance the weight is from the buttcap, and subtract 10 from it. wouldn't that make the weight added near the butcap negative? That's impossible. right? Basically, is there like an excel spreadsheet for calculating change in swingweight? Also, I'm still looking for the spreadshett that allows you to input values of weight added at the head and the but cap, or only the head, and it gives you the new balance and weight. thanks in advance

Yes. It will make the net distance negative, yes but since the swingweight is proportional to the square of this net distance, you'll always get a positive answer for your SW. It's in this thread.