The problem you’ve run into with metric threading on an imperial lathe is common. Your method of not using the threading dial and instead keeping the half-nuts engaged is the only practical solution. I use a handwheel for most threading for better control of the stop point and this also makes winding back to the start point easy.
Metric threading is a bit of a hassle even on a lathe with a metric leadscrew – they need interchangeable gears on the threading dial. The difference is that imperial threads are specified as threads per inch while metric threads are specified the other way around e.g. mm per thread. That is: distance per thread instead of threads per distance — and this subtlety makes all the difference.
Best book I’ve seen on threading is “Screwcutting in the Lathe” by Martin Cleeve. He explains metric threading on an imperial lathe and lots more – one of my favorite machining books. This jig for sharpening threading tools is from the book: http://www.gadgetbuilder.com/ThreadingTools.html