I was looking for a way to know when to increment the vol. parameter on the lists, and I noticed there wasn't any information on the site about volume to chapter mappings (or I completely missed it).

My suggestion is fairly simple:
To add an array (or maybe a dict) to the 'series' which holds these mappings.

For example:

(in JSON because it's easy to explain)

Internal representation:

[13, 24, 49, 80]

What a user sees:

Vol 1: Chapter 13
Vol 2: Chapter 24
Vol 3: Chapter 49
Vol 4: Chapter 80

or using a dict (which allows odd volume naming schemes)

Internal representation:

{"1" : 13, "2": 24, "2.5": 49, "3" : 80}

What a user sees:

Vol 1: Chapter 13
Vol 2: Chapter 24
Vol 2.5: Chapter 49
Vol 3: Chapter 80

I'm also open to other suggestions/alternatives.

"Status in Country of Origin" Series Field

"15 Volumes (Ongoing)" means that 15 volumes of the series are out in tank form in the country of origin. The 16th volume is in the works. Even if chapters for volume 16 are out in magazines, there are only 15 volumes in print! So the field indicates exactly how many volumes are currently printed and are possible to buy if so wished.

If it is a new series and no volume is currently released because all the chapters are in magazines, either leave the field blank or put "0 Volumes (Ongoing)". Do not put the number of chapters released. Only volumes!

For doujinshi, leave this field blank unless there are multiple volumes of a doujinshi series that use the same name.
For oneshots, they should all be "Oneshot (Complete)".

https://www.mangaupdates.com/faq.html?cid=7#item51

I also read EU/US comics and am a librarian.
Manga-Masters, My ANN-Lists + Imdb

https://www.mangaupdates.com/faq.html?cid=7#item51

Yes, I saw that, but that doesn't show how many volumes you would have read at Chapter X, only what the latest volume is (and not even what chapter that goes up to).