1) Put a gallon and a half of water in a pot and boil it for an hour. Then measure what's left, that'll give you your boil loss for an hour. There is no hard/fast number for evaporation as it's pot and boil vigor dependent.
2) Take the evap amount, plus the amount of water that will be absorbed by the grain (.75qt/lb), then whatever you want for either a no-sparge or a sparge volume.
Or use this for a ballpark:
If using dry yeast, you only need about 4g (1/3packet). I almost always overpitch to a half packet, it's easier to fold the packet in half and nip a corner.
Primaries: Hornet ESB
Bottled: K23 (last one!)
Kegged: Harvest Graf 2014, DPA Clone, Dampfbier
On Deck: All Brews Subject to Change With No Reason - loving small batch brewing AND Kegging!