surplus value calculations

I've read on here some in depth breakdowns of surplus calculations and was wondering about where to find this information.

1. I've seen Sickels at minorleagueball does the A, B, or C ratings but where do you get the 45, 50, or 55 values from?
2. For major leaguers, which projection system is preferred? Steamer, ZiPs, other??? or just use his 2017 numbers?
3. What $/WAR rate is currently speculated to be correct?
4. Any other information needed would be helpful as well as I don't understand the process of comparing players.

I've long liked to dream up trade scenarios and often find myself "out in left field" when reading responses that i'm out of touch with how this process really works. While trade deadlines and "the missing piece" scenarios can make any responsible GM alter his thought process, I'd like to be a bit more grounded when dreaming up these scenarios.

Thanks in advance!