Most recipes I've seen are for All Grain. Is there any easy way to convert the AG to a partial mash recipe? Should I simply take half of the grains and throw in 3-4lbs of Dry or Liquid Malt Extract? Keep the same amt of hops etc? Or is there a more complex calculation that I'd need to do depending on which grains were used?
The good beer software out there will do the conversion for you.
The way I do it is I try to figure out the brewer's efficiency (it's helpful if they list it; if not then I use 70%), and then multiply the pounds of grain by that number. The recipe you listed is already extract, so I'll just make up a quick recipe.
5 lb 2-row
1 lb wheat
0.5 lb Crystal 20L
0.5 lb Cara-munich
If you saw this recipe, you could multiply 5 lbs. by .7 (which is 70%) to get 3.5 lbs of 2-row extract (or just light DME). You would get 0.7 lbs of wheat extract - but remember, most wheat extract is some proportion barley. If the wheat extract is 50/50, then you'd add 1.4 lbs of that and take away .7 lbs of the light DME.
Then, I look at the specialty grains - the stuff that isn't basically 2-row, pilsner or wheat. Essentially, if you can't find an extract called that thing, then it's a specialty grain. You should partial mash those to get closer to the real beer, but if you don't do partial mashes, well... you won't get very close to the recipe.
The crystal and the cara don't have malt extracts named for them, so those are your partial mash grains. I feel that it's helpful to add in about a pound of 2-row to make sure the mash has plenty of enzymes and extra flavor, so the converted recipe, as
*I* would make it, is as follows:
2.1 lbs light DME
1.4 lbs wheat DME (50/50)
1 lb 2-row
0.5 lb Crystal 20L
0.5 lb Cara-munich
You would mash 2 lbs. of grains, then add the extract in towards the end of the boil.
That's just how I would do it; others would disagree.
Also, some recipes are for 10 gallon batches. I'm only making 5 gallon batches. Can I simply half everything (including grains, hops, water etc)?
Within reason, yes. It won't be
exactly exactly the same, but it will be close enough to not make a difference.
EDIT: bad specialty grain examples... neither of those need a mash, they can be steeped only. Still, I would tend to use an extra pound of 2-row, just to make the beer a little better, IMO. Also, if you substitute 2-row grain back in for extract, remember 1 lb of grain only replaces 0.7 lb of extract.