I know a lot of you are pass this point in their career but I will just post this up here because I realize there are so many young persons out there searching for this bits of info.
A little theory didn't hurt anybody so... These 3 methods are the ones I use the most in my daily job and they never failed me. That's why I said to myself that I need to compile them in a short instructive video everybody could understand without too much math and chemistry. I will also list them under the Instructions section just to leave room for extra discussion and to be able to post some other written explanations related to the topic.