Do you find that 0.004" / ~0.1mm isn't quite as fine a feed as you would like on your mini lathe?
When you want a really superb finish, use a pair of 100T change wheels instead of 80T with your 20T gears to get a fine feed of 0.0025" per revolution on an imperial mini lathe or 0.06mm on a metric one. Note that on some models of Mini Lathe you will need to modify or replace the gear mounting banjo to allow the larger diameter gears to be fitted.
Download this STL file for 3D printing. It is a 100-tooth 1-mod gear , 8mm thick with a 12mm bore and a 1.5mm deep by 3mm wide keyway. Print two and you now have access to a really fine feed.