I could be wrong when it comes to countertops but I was told that a linear foot was based on a 4' width while measuring the length. So, an 8' long countertop is 32 linear feet. It doesnt' matter if the width of the top is 3.5' or 2.5' or whatever. (So I was told because the 'waste' is what I have to pay for anyway so it must be include in the measurements, hence, the price too.) You count the waste.
As far as an L-shaped counter, I'd suspect you measure the '|' first, then the '_' part of the 'L' ONLY, where in the latter measurement, you measure UP TO the '|' of the 'L'; DON'T include it.) See what I mean??? As I said, I could be wrong but that's how it was explained to me when we had our tops re-done last Spring. Maybe someone else can reference you to a WEB site or describe it better.
Here's another thread on Linear Feet that may be of use. My best to ya and hope this helps.