Regardless of the scale, Stop converting! If your plans are metric, then get yourself a metric scale/ruler and work the dimensions as given on the drawings. If your model is European (or Asian) the drawings and materials will all (most likely) have been milled to metric dimensions.
When you start drawing your own plans, it will be time to worry about scale and which material dimensions make the most sense.
FWIW, I agree... with caveats. lol
I stand to be corrected, but I personally have never even seen a ship model kit that was measured in anything other than metric (mm's). However, if you're working from original ship plans that are in imperial units and scaling down for a model, then a conversion tool can be bloody handy. Especially since the vast majority of materials sold for model ship building are going to be in metric sizes.
IMHO, the more the OP begins to work and think in mm's, the easier it's going to become for him to visualize what 5mm looks like, thus making material selection faster and easier. If the instructions say you need a 0.5x5x300 walnut strip for second planking layer, you can tell by looking at your supplies which strips those are... sure, the walnut would be a dead giveaway, but I think everyone knows what I'm getting at. Plus, calipers that measure in mm make sizing masts easier, as well as doing a quick check if you're unsure that piece of square stock is a 4 or 5mm piece, or if that deadeye you need is a 3 or 4mm version.
Using the Amazon example, here are some links for you:
12" mm and inch clear flexible ruler
metric mm caliper
You should also be able to find metric rulers at your local Staples or other stationary store, and certainly check around your local hardware shops to see what they're carrying for calipers. The only reason I provided the links to Amazon is that they're typically less expensive, but if you don't want to wait a couple of weeks, then sourcing locally and paying a little more is the way to go. I like the bundles of inexpensive rulers because I can butcher them as much as I want for convenience (marking them up with a fine tip permanent marker, etc.), and you can find bundles of 30 for around $10. Cheap like borscht.