This is what I do, but I'm a high level ... and my killgem starts out with an 80% chance of a crit.
Damage Rating : The result of multiplying the killing gem's maximum Damage by its Critical Damage Multiplier. Using White to start off with, instead of Black, in psorek's 32 spec killing gem, with 6 pure yellow amps, this usually starts off around 240,000 or so. It grows considerably over the course of a farm.
Wave 1 through 3 : Since I'm using G1 gems to bomb waves, monster armor is skyrocketing compared to monster HP. Since Bolt has not charged yet, I bomb these three monster waves so that monster HP + monster armor is as close to the Damage Rating of my killgem, without going over.
Wave 4 through 6, and beyond : Still using G1 gems to bomb, which I use until I've enough mana left over from upgrading gems to comfortably bomb with a G5, or G15, gem. Bolt usually charges around this time, though, so I put it on the killing gem. Since Bolt means the gem ignores armor, I stop counting the monster armor, and I bomb so that monster HP is close to, without going over, the Damage Rating.
When I am using G15 gems to bomb, I start paying attention to the monsters at the killing trap. When the last of the really high armor monsters is killed, I switch from Bolt to Barrage. Depending upon the waves and the map, that has happened as early as Wave 12 and as late as Wave 20. Be careful! A decently developed killing gem still won't be able to penetrate e22 level of armor all that well if its Damage Rating is in what would be the e17 levels (if e levels started that low).
As for when to move from one angering gem to the next, that's simple. Grade 1, 5, 15, 25, 35, 45, 55 .. it all depends on how much mana you are sitting on in your mana pool after your gem upgrading. However much mana you have left, you want your angering gem to cost two "illions" less. Trust me, its easier to do than to explain ... if you have 20,000 mana left after upgrading gems, then you still want to use G1 gems. If you have 500,000,000,000 mana left after upgrading your gems, then you want your bombs to cost around, but a little under, 1,000,000 mana (two "illions" less than trillion is million). If you are sitting on 2,456,754,234,000,000,000 mana (two quintillion) then two "illions" less would be a trillion, or 1,000,000,000,000. Regular gems, at max levels, cost just a little under those amounts at Grade 5, (1000), Grade 15 (1,000,000), Grade 25 (1,000,000,000), Grade 35 (1,000,000,000,000), 45 (1,000,000,000,000,000), and so on, all odd multiples of 5.
Make the transition as soon as you can, in order to both maximize your exp per wave, as well as to get out of the early phase where monster armor is humongous compared to monster HP.
One reason I've not put that into guide format is it is based on high level experience, and I'm not sure of its applicability at low levels. I developed that multiplication method out of a simple desire to know how much I should be bombing in my own games, if anyone else has any better ideas, I'm all ears.