Top off to the total volume you want in the fermentor. Aerate/mix very , very well. Take your OG reading.
If you haven't mixed the boiled volume and your top off water sufficiently this reading could either be high or low. Higher OG if your sample came from and area of high sugar concentration in the fermentor or low if from a low sugar area.
If you can get the boil kettle volume lower than 80°, do it. Pitching your yeast in wort cooler than the fermentation temperature is better than trying to cool down a fermentation that starts to warm. Less stress on the yeast.