For some people who follow the rules and paths, such as when building/making some things, learning to get "off the road" and make your on path can be confusing and or frightening. Decisions become hard to make.
Wood moves, techniques differ, learning to make the adjustment on the fly is necessary.
Above in the different sections of IAP, in the middle blue row, second from left is the Library. In the Library under "Reference" are different charts that reference, metric, inch and fractions. I have a 6 page chart with fraction, metric and inch references on the wall in my shop near the lathe.
1. Get one of those charts. I Like the MS Word Doc "Drill Bit Conversions".
2. Get a set of HF (Or other store's) 115 piece drill bit set.
The drill bit set is not my main drill bits for pens but I go to them often when I need to increase the size as mentioned by Ed above. I probably use the HF bits about 25% of the time to increase or decrease a hole size by tenths of a mm or 1/100th of an inch. (I prefer mm measurements on pens even though I use fractions and inches in flat work.)
For easy to crack/fragile wood/material, I drill a hole .1 or .2 mm or even .3 mm larger and use rubber epoxy. The larger hole allows for expansion and the rubber epoxy is hard enough yet flexible enough to allow for expansion/contraction that CA and normal epoxy doesn't.
Tenths of an MM larger is not always a problem. By measuring with the calipers the nib, clip end and ring sizes, and the tube sizes, you can easily guess how much "play" room you need or and have.
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