Is this strange or normal?

Good catch. In fact, I intended that last field to show comparitive, BTU, for example, if there was a 10% reduction that field would show 90%. I could rework it, for now just reworded the description, hopefully clearer:



If you try the spreadsheet, enter 0 in percent of ethanol field, you'll see the bottom percent go to 100%.

 

I also put "=100%-B5" in B6 so you just have to enter a % Ethanol value in B5 to make the B6 value change automagically.
(We are such nerds...)