Thick latex flatbands

So I got a Gym In The Pocket exercise band. The band is <1.5mm thick and is made of latex. Very stretchy, out to over 300%. I made two sets out of it, each set is double layer(10inches in length) so there's 3mm of latex flatband on each fork arm. I set it up on a Barnett Black Widow slingshot and it shoots pretty accurate, considering I havent practiced in a while now. The draw weight is approx around 20lbs at 26'' draw length, I draw to my eye. Im a little concerned about the killing potential of this setup, I dont have a chrony so I cant be certain how much power it puts out. I use a .38 cal leadball sinker, and at 25feet it will put round dents in hardwood so what do you think of this setup. Any idea on testing its power/potential? thanks