Hey, Slink,
That is a very inovative idea, that dental mirror. May even work about as well as the wife, which is the tool I use to check the oil.
The larger issue remains: what idiot puts a dime sized viewing port down under the bike where even on a good day it is hard to see, and almost impossible when there is any reflection on the sight glass? Whatever happened to "dip sticks," or do they all work for the Triumph design team? To be fair, if you have to have a sight glass, why not make it an inch or more in diameter, with a line or two on it, like most of the Japanese bikes do?
What I don't understand about Triumph is how they can do the really big things so very well and then leave us with a lot of little niggling agravations (sp?) to hound us every day: like the sight glass, removing a seat on a Tbird; having to take the bike apart to check the plugs; having to use shims instead of wrench adjustable tappets on the valves. The list goes on and on.
Meanwhile, back at the original question. You DO have to "level" the bike, as noted above, if you check it on the center stand, or you will get a false overfull reading. And, on the 885 Triples if you fill above the sight glass, and you WILL if you put the amount of oil in the manual recommends you will very likely end with oil seaping out of the left side and making a little "traditional" Trumpet puddle on the garage floor, to help remind us all of the good old days with our 60s and 70s Bonnevilles.
Take care,
Monte