I'm confused by this thread's seeming obsession with assessing the visibility at DA.
The only time you're interested in the numbers, be they met vis, corrected met vis or RVR is BEFORE the meat of the ILS is commenced, i.e. is the visibility at or above the minima required to make the approach? Answer 'No' = different approach, different vis (if you can hang around that long) or different airport/runway. Answer 'Yes' = conduct approach.
Most countries now have the Approach Ban Point (ABP). This is variously, depending on where in the World you are, at the FAF, OM or 1000'. If the vis is below minima at or before the ABP, discontinue the approach. Vis above minima, then continue the approach. Vis decision made......no more discussion....no light counting.
At DA the only decision that needs to be made is 'can I see the elements required of this type of approach'? These have obviously been designed such that a safe landing can be carried out and I won't insult your collective intelligence by listing them (as well as can't be arsed to list the 10 options of a CAT 1
).
So, to the original question. At DA you don't decide if you have 800m etc visibility. That was done ages ago or you wouldn't have commenced the approach. At DA you either land or go-around.
(For the pedants; yes, I do realise there's no DA on a CAT IIIB/C, so no decision needs to be made
)