Zones, bay floor area

Felix you are correct with the ZFA. The reason for the difference between sidelighting and the toplighting calculations is because there is a big difference between how sidelighting and toplighting actually works. If such a method were to have the same means of calculating sidelighting and toplighting, it would be invalid. The view, or access to the sky vault is completely different between the two scenarios.

I do not know of any tutorials that show you how the areas are calculated. I suggest you personally consult with someone who knows how to correctly document this credit.

Felix,
The "Sample Prescriptive Compliance Calculation and Path" file in the Documentation Toolkit tab above may help answer some of your questions.

The graphics show how the floor area zones are set up and then adjusted to first determine how much daylight coverage you get from side lighting. You may want to do all the side lighting calcs first for all the rooms to see how close you are to 75% of your regularly occupied spaces.

Then, look at any areas in a room that don't have enough daylight and assess whether you have toplighting over those areas. If you do, analyze the additional daylight provided by skylights for those top-lit areas that are below the skylight as defined by the footnote #5 (using ceiling height, distance to next skylight or partition, etc.)

Since it would be possible to over-estimate daylight provided if you add the lighting provided by sidelight and toplight for the same floor area, there's a note at the top saying "only one daylight method may be selected for a space." I think most users assume that Prescriptive section in columns M-Z is one "method," but my understanding is that this spreadsheet can only analyze the light hitting an area of the floor from one source - either all from the side or all from the top.

This might be clear as mud... if you're confused, look at the graphic for Step 6 in the LEEDUser sample above. This example doesn't use the 2009 spreadsheet - it uses a simpler scenario to manually calculate the areas being day lit. Notice in the example how an area that has both side light and top light doesn't get double counted since they are looking at the % of the space that is not-day lit - 550 sf. But in the spreadsheet, I don't think there's a way to distinguish in one space (one row of the spreadsheet) the areas that are have only top light, only sidelight, or a mixture of the two.

The work around might be to define smaller spaces or zones based on what kind of day lighting they receive- say, a perimeter zone of a class room that only gets sidelight, and an interior area that's all top lit.

This prescriptive method has used different calculation methods in several different LEED versions, and has always been an imperfect tool. To make it more accurate would make it even more complicated... so for some situations it just won't give an accurate assessment of the daylight. If that's the case, you'll have to use either the Simulation path or the Measurement path.
Hope that helps,