I bought an UTG 4-16x44 OP3 scope for the PCP air rifle I'm waiting for. It focuses to 10M. I want to focus to 24 ft. I have a close up lens for my Leopold scopes that gets my scopes to focus a bit closer then they do without it. Right now I'm working on 3d printing an adapter so I can screw it onto this scope once I figure out thread specs.How do I calculate the fractional diopter for a close up lens and where can I find unmounted lens for sale at a reasonable price? Thanks