window-to-floor-area: including frame?

Just the area of the transparent glazing.

I have always done just the overall window rough-in dimensions.

The reason you cannot find any official statement on this is because why would you take credit for opaque area which does not provide daylight to a space? You can say its against the intent of the credit. Is there anyway in which a reviewer could determine whether you are doing this or not?, No, unless you provide CAD files that can be measured.

Well if you are using the opaque elements of the window such as mullions and frames, then you should be reducing the VLT you are using because you are taking credit for an area which is not providing daylight to the space. Modeling protocol would be:

 the area of the sills, jambs, and mullions can be calculated and summed into an ‘occlusion area’. Then the net window area can be calculated and the ratio of the occluded area relative to the rough opening determined (occlusion area/gross window area = reduction factor). Then, the gross window area can be simulated with the reduction factor applied to the VLT of the specified glass. (E.G., If the gross window area or rough opening is equal to 100 square feet and the occlusion area is 5 square feet; a 5% reduction factor is applied to the VLT of the glass. Therefore glass with 70% VLT X 95% = 66.5% VLT.)
 When window framing details are not known, a 20% reduction factor should be assumed relative to the rough opening. For skylights, a 10% reduction factor should be assumed relative to the rough opening.

Would a typical mullion of like 1 inch make a difference, probably not, but larger frames of store fronts and curtain walls it would be.

Not to worry. No one is trying to "take credit for opaque area which does not provide daylight to a space."

If we can all agree that: 1. LEED is not the authority on daylighting, 2. the prescriptive method is intended as an easy, cheap way to determine credit compliance and not daylighting aptitude (an important distinction), and 3. the prescriptive method is crude, then I submit that it is not the intent of the prescriptive method that the design team spend their time calculating the area of every window pane, but that it is assumed by the creators of the prescriptive method that all windows have frames and mullions.

This forum is for LEED users. My statement is intended to help users use LEED. Reviewers have always had the option to review my calculations with drawings or to ask for more drawings to do so.

This is similar to the confusion over the simulation option and what it means to meet the minimum footcandle level at 3pm and 9pm. Even though it seems to me that the 75% should be met only with points that meet at both times, reviewers haven't been responding that way. It is all in the interpretation. I'm letting people know that reviewers have been responding to my interpretation that a certain amount of occlusion is already included in the prescriptive method.

I'm not sure where the 20% for windows or 10% for skylights comes from? Can you enlighten me?

I don;t worry whether people take credit or not. Much of the information provided must be accepted because we are all professionals who put our names on the results. Just like no one ever manipulates simulation parameters or other tweeks which cannot be asked for by the reviewer unless there is an unquestionable issue that has been submitted. It must be assumed that everyone is doing it the right way and are honest. There are a number of projects that have earned the daylight credit over the many versions that are very far from what many of us in the industry would consider well. if even daylit. But their case studies spout that their building is daylit and has earned the credit.

I agree with all you said, x4.

Reviewers don;t really have that option anymore of just calling out items that look questionable and asking for documentation that is not required. The days of credits being audited, just to be audited are in the past and to get documentation that is not required submitted needs to have pretty definitive case presented to get it.

Jill, the info i submitted is from a draft of a document that will probably be published here within a few months and has been the work of many daylight industry practitioners to create consistency across the board. I can;t specifically say how the 20 and 10% was established since i was not one of the contributors. But i'll try to find out.