Jeez, I even show the math, and still get contradicted.

Nope, 100MB is 800000000. Your link didn't work for me, but I found the working one through Google. It uses 1024 for the multiplier in the calculator, and is thus incorrect. This seems to be an old page. Though it mentions the IEC standard, it states: "The new IEC International Standards, which are not commonly used yet...". In fact, the standard has been in common usage for many years. There are any number of MB calculators on the internets. Here's one:
Bit Calculator - Convert between bits/bytes/kilobits/kilobytes/megabits/megabytes/gigabits/gigabytes.. Change the Notation Type to '"kilo is 1000'. You will note that this page also has a link to the National Institute of Standards,
Definitions of the SI units: The binary prefixes.