id say it is the 1000ftAGL point.
I dont see the purpose in starting the approach when the WX is below the minima as it will most likely be the same when you reach the OM(Precision app)or 1000ft AGl(Non precision app)..
But legally I believe you can start the approach if wx is below minima but go missed if by the om?1000ft agl the wx remains below the landing minima.
However if you flew passed them and the wx gets below minima..you can continue the approach until DH/MDA..if by lets say 200ftDH you see some approacg lights..you can drop another 100ft(DH becomes 100ft)..
Of course i stand to be corrected by the moderators..
M.85