I press the string at the first and last fret and see how far away the string is from the middle frets. Personally I like it so close you can't see any gap but just about feel a tiny bit when you press it down. It's all by feel, so just try it and see. Acoustically, my basses with low action rattle a fair bit but plugged in you get that nice clank when you dig in and no nasty fret buzz.
If the saddles won't go low enough my preference is to file down the string saddle using abrasive cord rather than shim the neck, unless it needs adjustment by shimming one end, but in that case I'd probably get rid instead. Ive had many instruments that can't support a low enough actin and the next owner they suit fine!