That little formula for manafarm and killing gems is really only something simple to get started with. Later on you are going to want to use what psorek came up with and put in his High End guide (check the useful links sticky).
While you could use all G2's in those simple starting gem recipes, there is little reason to. If you want to start off with a more expensive gem, you would be better off doing something to supergem the base gems instead of simply starting off with higher grade base gems. You should come off with pretty much the exact same gem if you were to combine 16 G2 gems in the ration I suggested above, or if you combine 16 G1's and then U the result anyway.
When making starting gems, you will eventually want to supergem them - make them stronger stat-wise, though a lower grade, than the equivalent cost gem. For example, if you were to make a G3 Orange gem, it will cost X mana. You could also take 4 G1 Orange gems, combine two into a G2 gem, then combine the other two, one at a time, into that G2 gem, resulting in a G2 gem that costs what the G3 costs, but is stronger. The lower the grade you start to supergem, the stronger the gems eventually become.
If you have the initial mana to spare, after almost maxing out those skills I suggest below, then you will want to start toying with psorek's supergem methods. He details a powerful initial gem for both killing and manafarming, as well as a pair of upgrade plans (16 and 64) that will make your gems more powerful than their cost would imply. For what its worth, U is the simplest upgrade you can do, and is the least effective. You won't actually need to use those initial 32 count gems, or the 16 or 64 gem upgrade plans, but when you can and do start using them, you will find your efforts at manafarming much more effective.
As for your skills, I cannot give you hard numbers. True Colors, Masonry, Fusion, Mana Stream, Resonance, Amplifiers, Fury, and Traps are all useful skills, as are the gem skills for White, Black, Red, Yellow, and Orange. If you need to skimp, I'd skimp on the Mana Stream - ideally most of your mana income comes from the manatrap, not killing the monsters, and you don't really need any given mana pool level, just the mana in the pool.
Try to keep all of those skills at a multiple of 3, but you do not necessarily have to have them at the highest multiple of 3 you can reach. I do not know what level your skills cap at, but if they cap at, say, 28, you do not necessarily need to have those skills all at 27. If you can get away without the initial mana, go for it. If not, then knocking them all down to 21 or even 18 may help you out even more. The reason for "multiple of 3" should be fairly obvious - its the secondary benefit to the skill. The one skill that you can really afford to have lower is, as already stated, Mana Stream.
I use White in the slow gems for a simple reason : I'm not raising those gems to high levels at all. The White boosts both the slow% (to its cap) and the chain hit by a decent amount. The less mana I need to spend on the slow gem, the better. And it hits everything that crosses it due to the high firing rate of traps and the moderate chain hit. If your gem does not max out the slow percentage in the trap at Grade 2, then just hit U a few times until it does, then forget about it. It will hit all the monsters you need it to. There is no need to supergem the slow gems more than the initial formation, they do their job well enough without it.
