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Some people simply express the imperial sizes as metric but that won't be much help to use as you want to use existing metric tools and materials and I don't tend to consider that a proper conversion as it's simply working in different units.
Jo, did you miss the point about wanting to use his existing metric tools? There are a few items, the two I mention for example that would be PGMS or silver steel into reamed holes. As the OP says he already has metric tooling then surely it is better to use stock that fits into a hole formed with a 10mm reamer he has than buying in a 3/8" one
Or why turn down readily available 5mm nonimal stock to 0.190" to put a #10-24 thread onto it with dies he does not have when using readily available 5mm stock and M5 die that he has will do
5mm is not a standard stock size, 4mm or 6mm is. Metric threading tooling is are easy to come by as is BA rather than foreign ones which in the UK you can't get easily.
Achim (fumopuc) did a metric version here:https://www.modelenginemaker.com/index.php/topic,6835.0.htmlI expect he will comment later