Agree with Gavin completely, as usual. One more thing on drilling holes for centers. All drill bits are not the same. If you buy say an M12 drill, chances are that it is slightly oversize. Especially if you buy the usual woodworking bits. They are intentionally oversize. This is so's you can push a nominal 12 mm dowel through it. The dowel itself will very wildly, but that is off-topic. So my practice is to measure the drill before I do the hole. I also measure the center itself. Steel rod is a lot more accurate than a wooden dowel, but still, maybe it was Monday morning at the factory

. A machinist's tight "press-in" fit.is .002" (about .05 mm) in steel. That is, the hole should be .05mm smaller than the rod. In wood we cannot get these accuracies, so we settle for 0.1mm. Or even less depending upon species; pine can be forced much more than, say, ash. Now most people building pole lathes are lucky to have bits in even millimeter sizes, or the usual Imperial fractional series. So what do you do then? I say epoxy. If it is a sliding fit, or even a mildly fiddly tight fit, put some epoxy on it. Make absolutely sure there is no trace of oil on the center. Rough the shank up with a coarse file, or a cold chisel. (Not the point!) Make sure the hole is as clean as can be, no chips or sawdust. Let the epoxy cure at least overnight. Epoxy is wonder stuff, but oil defeats it completely. So do chips, sawdust and other debris.
Which brings me to the subject of measuring drill bits, or steel rod. For this purpose a ruler is of no use. What you want is
vernier calipers (or callipers. both spellings common). If you want to be fancy you can get a pair of digital calipers that will read out to .0001" or 20 microns, will work metric or Imperial, can use it as a scribe, hardened tips, wonderland ... but it will cost you about 25 quid UK per Axmister. For one pole lathe? Not worth it. For years I made do with a pair of plastic vernier calipers made in China of course. The big one can hit .05mm and less by interpolation. Perfectly good for bodger's work, the little one good for miniatures, I still use them even though I use the digital brethren for machine work. The plastic calipers cost me 0ne Dollar. In Britain you can also find these things at car boot sales really cheap. Made in metal, too. You will have to learn to read the vernier scale. I learned this in High School in Venezuela and I am sure you can too. Lots of stuff on the net to get you started. The dial calipers are good too, but actually the vernier scales are more accurate, if somewhat harder to read.