first off i want to know if you know how cool this spreadsheet is...
Thanks! It was originally developed for my own use, but after a few people asked for it I just posted it on the web site.
second: PPG = gravity units(right?)
Yeah. For each source of fermentable sugar, enter the points per pound per gallon of wort. (For mashed grains enter the theoretical max; this automatically gets scaled back by your mash efficiency, which you must also enter up above.)
third: boil points?
That's part of an internal intermediate calculation; don't enter anything here, let the spreadsheet calculate it. Wort gravity affects hop extraction, so the spreadsheet needs to track which sources of sugar are boiled with the hops and which are not. Sugar from all your mashed/steeped grains is assumed to be boiled, plus any malt extract and other sugars where you've entered a "Y" in the "Boiled?" column.
AF stands for "age factor"; it is a rough approximation of how much bitterness hops lose per year (assuming they're stored frozen). You can adjust this if you like; the value that's in there ( 0.8 ) is a compromise value which (IIRC) was derived from data in Ray Daniels' "Designing Great Beers" book. This is used to automatically adjust your estimated IBUs based on the numbers you enter in the "Age" column of the Hops section.
BF is calculated from the Boil Points. Let the spreadsheet fill this in. (It is another input to the IBU estimation calculation.)
You may have run across references to various methods of hop IBU calculation (Tinseth, Rager, etc.); the calculation used in this spreadsheet is essentially Tinseth's formula, modified to take age of the hops into account.
Given the proliferation of online brewing calculators and smartphone brewing apps, this spreadsheet is becoming a bit of an anachronism in that it doesn't automatically fill in the gravity points or color of the various malts for you; you must look those numbers up and enter them manually. I (and a few other people I know) still use it because it's simple to use and works quite well once you understand it.
Caveat: The color calculation tends to be a bit off (it tells you the beer will be darker than it really will be) for darker beers; I'd take the color numbers it spits out with a grain of salt for anything higher than around 20 SRM. Fixing this has been on my "to do" list for years...