I do see your point, but there is a definitive amount of juice you're going to get from a lemon and you can look up for example :1 lemon equals 1/3 cup of lemon juice. That's what I was hoping for here. Perhaps I'm way over my head. Like I said, brand new to all of this. @_@
It is the same for extract, except more so. You can look up the PPG for an ingredient (points per pound per gallon) and calculate its exact contribution to the SG from there. e.g. see here:
http://www.howtobrew.com/section1/chapter3-4.html
For extract, the PPG numbers are the PPG numbers, there is no getting around that. For a mash, it depends on your efficiency. Typical efficiency is around 75% from what I understand (I'm an extract brewer only so far), but it can range much lower or higher depending on process.
Check out
http://www.howtobrew.com for starters, it is an online version of one of the most popular (and best) homebrewing books. Be warned, though, this is the first edition (it is now on the 3rd in print) and there is a little bit of outdated information in here.
If you had never fermented before, I'd be suggesting
The Joy of Homebrewing by Charlie Papazian. It is neither as clear nor as comprehensive nor as concise as the Palmer book (
How to Brew), but it will make you feel confident about brewing. The first few times I started reading the Palmer book, every time I was look, "Oh crap, I screwed up soooo many things on all of my beers, what will I do!!!" Papazian is much more comforting
But since you say you've done fruit wines before, you probably don't need the confidence boost, and in that case I'd recommend Palmer.