F-16 flat band starter slingshot

I only recently joined the forum and have received lots of good advice on which design to get for my first foray into adult(ish) slingshots. Instead of ordering a cheap Chinese sling, my nephew told me he had an older F-16 that had been retubed with some mystery surgical tubing with a really heavy draw. After watching Simple Shot's video on using flat bands on wire frames, I decided to switch it over to flat TTF. I didn't have any of the Gold Theraband frequently mentioned, but have rolls of red, green, black, and grey in thicknesses of 0.21mm, 0.24mm, 0.35mm, and 0.54mm. After playing with strips of each, I settled on the grey/0.54mm. I tied up the old pouch with roughly 8" active length of grey non tapered at 25mm width. I'm probably getting 400% elongation. The bands feel like they're starting to stack up around that number. Tapered would probably work a bit better, but I'm just getting started and will experiment when I get a good assortment of GZK bulk band. All I had handy was marbles so I started at about 7 yards. I'm not any sort of deadeye yet, but my first 20 shots stayed in about a 10" circle. Since I'm shooting upstairs at my office ???? I wasn't able to play very long. I was really surprised how easily it shot and the velocity and energy it generated. It would absolutely humanely work for squirrels. I was planning to buy a Hammer or Scout and probably will at some point, but I have access to just about any material so my new plan is to shoot tons of marbles until I begin to get a feel for accurate instinctive shooting. I also went ahead and cut tanned deer hide (pliable, but not stretchy) pouches and a bunch of replacement bands so this should keep me occupied for quite a while. I'm sure I can improve my band attachment skills, but I'm actually really excited about this. Thanks again for all the advice and I'm welcome to any suggestions. Thanks.
Charles