I would recommend investing some $10 into this book:
https://www.wheelpro.co.uk/wheelbuilding/book.php
That is the simplest way to learn how to build wheels.
If you wish to learn why, I suggest you read the Jobst Brandt's "The Bicycle Wheel".
The book's author, Roger Musson, made a spoke length calculator that has worked very well for me with both bicycle and motorcycle wheel building (with some tweaks for the latter as the calc. doesn't give any support for 3+ mm thick spokes).
Unfortunately, the calculator is no longer supported for those who haven't paid for the book.
With the spokes being too short, the nipple head might break off with heavy/strong riders even when strong rims are used. Case in point:
https://bike.bikegremlin.com/8431/custom-wheel/#4
One of the most common mistakes when calculating spoke length is relying on the manufacturer's ERD data.
The data provided for the hubs is less critical and more often accurate, but the ERD measuring methods can affect the results (and if the calculator you use doesn't use the same method, you could run into problems, as you have).
What FBinNY explained is a good, reliable way of measuring the ERD.
Longer explanation with pictures:
https://bike.bikegremlin.com/9266/ef...-rim-diameter/
Still, I think investing the money and time into the above-mentioned book(s) will probably help more than a thousand forum questions and replies.
I can not recommend those enough.
Relja