It is something you picked up cheap and it is your little "project", make it work
You've made a good start in getting nice concentric rings, as Brian suggests nothing beats a star test under stable skies at zenith.
One thing I have read on Cloudy Nights is the use of a 33% aperture mask and I believe a Cheshire Eyepiece but you'd be better off researching it. You have a perishable lens, you may as well make it your learning lens to see if you can bring it back from the grave. Worst comes to worst, throw the lens' and keep the tube
My collimating experience with telescopes is a newt and Glatter laser, focuser with a Glatter and star collimating a SCT than only needed very minor adjustments. Refractor lens', I'll leave that to you!