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.
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.