A. You want to check down to what temperature that dilution covers you for. You don't want it to freeze up on you. Depending on where you live -10C would be the minimum - could be more -20C or more.
B. I found the best way to very roughly measure what level is in the bottle (i.e. a lot, some, very little) is to get some clear 4mm air line tubing (say a metre),wriggle it in as far as it will go into the bottle, hold it close to the top of the bottle(to reference the top of the bottle), put your finger over the end not in the bottle, draw it out and see where the liquid is in the tube OR (much simpler) pour the liquid in very slowly until you just see it in the neck of the bottle (the secret is to pour it in slowly!) and then stop (another secret!).