Interesting shot. Well done! Solid A !
The f/20 gave you good depth of field! The lighting was perfect.
Since you mentioned you are a beginner, below are some things to think about. Your photo was really good, so don't take them as criticism!
If you had shot that at f/8 it probably would have been a bit sharper but you would have had to rotate to your left and get all of the moth straight in the focal plane. Easier said than done! I practice using a small ruler and when you have the entire ruler in focus...bingo!
I try to shoot my macros in a variety of settings (if the critter isn't moving) and see what worked best, and evaluate why one shot was better than another. The smallest adjustment can make or break your focus and sharpness! I try to use a monopod or tripod most of the time to eliminate any camera shake. I really try to shoot that lens at a minimum of 1/100 of a second, the faster the better to minimize any movement. If the flash supports High-Speed Sync you can go a lot higher!
Of course higher shutter speed, higher ISO, but the Z's really handle higher ISOs well and Topaz DeNoise (or the setting in Sharpen) can really be your friend.
The ring flash worked great! What ring flash were you using? Lighting is always an issue!!!
You might also see what you get if you run the image through Topaz Sharpen. It can give you some really amazing results. Using Lightroom's new masking features would probably have worked well with this image, or Photoshop's sharpen tool (which I use very sparingly).
Great image and keep on shooting. Great time of year for macro!!