<A href="http://www.forestry-suppliers.com/t01_pages/tt_pdf/1211_ScaleStick.pdf"> here</a> is a PDF document with some instructions on using the 3 most common scales.
If you take the diameter of the small end of the log divided by the square root of 2, you'll know approximately how big a square will fit in the log with no allowance for wane.
say your log is 17"
17/1.414=12; you can get a 12" cant from a log with little wane.
If you're cutting that cant with a chain saw mill and 3/8" chain, your kerf will use up 5/16"; 8 cuts and you can have 9 boards each a hair over 1"
If using a bandsaw mill, you'll get at least 2 more 1" boards out of that log, and if using an insert tooth circle saw mill, figure your kerf at 1/4", cut your cant at 12" x 12-1/2" for 10 boards. (Actually, your cant would only need to be 12-1/4" but most of the old portable mills weren't that accurate so old-time sawyers would add...)
A board foot is a 1" board, 12" long and 12" wide. After drying and dressing, that board is 3/4" thick, 11" to 11-1/4" wide (depending on species, mill, and coast) and 12" long. There's 2 bf in a 2x12, 1 foot long.
Linear feet is running feet. If you took your 9: 1 x12's out of an 8' log, you'd have 72 linear feet of 1x12 lumber. Linear feet is usually used in measuring millwork or narrow stock.