@KrisD15 's methodology makes perfect sense & I completely agree with it. Thank you.
A warning to those who may be viewing this post - TurboTax seems to do it differently.
Although I would love to have a formula to use (to put into a spreadsheet) it dawned on me that such a formula probably doesn't exist, and that all the IRS provides is explanatory text. But, a specific formula may not be necessary anyway if all one needs is a reasonable method to demonstrate in the case of an audit.
