oh my guinness

It's a stew night tonight, and one day late, we're having beef and guinness stew. It's a long, busy day today, so I'm going to bring out the slow cooker - and adapt my recipe to suit. According to my Margaret Fulton Crockpot Cookbook virtually anything can be cooked in a crockpot - you just need to adjust the liquid levels as it won't evaporate while cooking. So my recipe calls for 1 cup of guinness and 2 cups of beef stock... Margaret reckons that one cup is enough liquid... Hmmmm. Do I go half a cup of guinness and half of beef stock? What do I do with the leftover guinness? Freeze it perhaps? Or, do I double my recipe so there's more meat and then freeze the leftovers? That'd be the sensible idea wouldn't it? Oh, and thrifty too!