Do you have a well-fitting suit coat or sports coat you can measure? There is a range of acceptable measurements, and I like mine to be about 2 inches larger than my own chest, so for you that would be 22.25". But different people have different preferences for how they like their clothes to fit.

 

Agree with Matt that measuring something you already own that fits well will give you the best chance at an internet purchase. The shoulder measurement is just as important. For me I am best with an 18 3/4 shoulder and a 45 inch chest with a 25 inch sleeve. Usually that equates to a 42R. But not always.

 

The problem with looking at the tag size when shopping eBay is that there’s no way of knowing the nature of any previous alterations.

I believe you alluded to something like this is another thread.

As for methods for “nailing the size”, I agree with the above.

 

The problem with looking at the tag size when shopping eBay is that there's no way of knowing the nature of any previous alterations.

I believe you alluded to something like this is another thread.

As for methods for "nailing the size", I agree with the above.

I did. And what's odd is the jacket that fits me well seems to measure 21" pit to pit and yet when I measure my chest I'm ~42.5".

Makes no sense to me.

 

As SG_67 said "tag size" doesn't matter except to narrow down your choices. I am typically a 46R in standard men's clothing but with so many different cuts and the slim craziness none of that matters. I have a 47 inch chest and in a regular fit jacket a 46 used to have 4 inches of extra, nowadays 2 inches is considered normal. I've even seen several brands that now list a 46R jacket with a chest measurement of less than 46 inches, I can't begin to understand why the jacket would not at least measure the listed chest size.

Like said above, measure a good fitting jacket you already have following these instructions:
https://askandyaboutclothes.com/clothing/style-tips/how-to-measure-a-jacket/

This process is followed by most of the professional dealers on ebay. Sometimes you get someone who doesn't follow this but lists his measurements the same way and it's off when you get it. Take their reputation as a seller and number of transactions they've done into account, also always buy from someone who allows returns. I got a jacket from a non-dealer who measured everything but the waist correctly on a jacket so now I have a great looking jacket that fits awesomely except I'm going to have to have like 4 inches taken in on the waist if it can be done.