When you use the var. ND filter and you guys who are shooting in really strong ambient, I'm assuming you are using high power manual strobes, so are you guys just going to sync speed everytime, F2.8 or your choice, then use ISO to fine tune the ambient you want if needed by chimping? Now at that point how are you getting proper flash exposure, for you guys with flash meters are you just setting your meter to 1/200 and test firing your flash until you are dialed in to a power that your meter kicks back as F2.8?
Whether you are using an ND filter or not you need a strong flash to compete with the sun. Using an ND filter doesnt require any extra flash power as the balance between sun and strobe power is the same, you are just making the camera use a different aperture (or different shutter speed with the same aperture) with the ND filter. If you have a meter (handheld or in camera) you dont have to chimp as much as you suggest. Set your ISO as low as you can and your shutter speed to max sync. Meter the scene. Decide how much you want to underexpose the background, if at all. Then dial in those settings. You can do this with the filter in place, or without and then just dial the aperture back the number of stops corresponding to your filter's density. Then add flash. If you have a flashmeter you can adjust the ISO in the meter to compensate for filter factor or just dial figure it in your head. If you dont have a meter chimp until you get the flash levels you want.
