Joe,
Easiest thing to do is measure and reference the chart you provided a link for which is what I do. After a while you can remember and Eldredge #5 is a size 16 or a New Home 1/2 is size 14 or a Singer 1 is size 16 but why try to remember all that. I have books that these numbers can be referenced but this depends on needle manufacturer and era so not very dependable. For me easiest to take out cheap calipers and measure and round up as measurements are not always dead on.
Jon
edit--If the needle is not present to measure sometimes you have to go by old charts and use best judgement. There are quite a few sizing systems. Some I have are Excelsior, Blelock, Free, Brewer, Boye, National, New Home etc.. The systems were meant to be confusing to inspire needle loyalty.
Last edited by jlhmnj; 12-28-2014 at 08:06 AM.