It can be less than that, but that simple formula is accurate, or accurate within any amount you should care about, for most scopes. However, it is more likely to be INaccurate with scopes that have magnification ranges wider than 4x, so while you can start out with those numbers as an assumption, if the values are critical to you I would wait for the published specs* or contact Vortex and ask.
*published specs can be wrong too, of course. And especially when they are at some retailer's website vs. the manufacturer.
Cool, thanks. Yes, I'm looking for accurate info for comparison. Hopefully someone will also have first-hand observation data to compare with, eventually. The simple formula is very helpful now, while we wait for official data.