A couple of suggestions.
1 ) If you can, start using psorek's 32 spec killgem. If you cannot, for whatever reason, then try this very simple gem made from all Grade 1 gems : 1 Red, 7 Yellow, 8 White.
2 ) Use Yellow, not Yellow/Red, in your Amplifiers.
White is actually stronger (a LOT stronger initially) than Black as a gem booster, up until Grade 30 or so. If you are going to be getting to higher than that Grade, then it may be worthwhile to start off with White, then farm hits with a lower Grade Black version, with the intent to replace the White with it when it is more powerful. Additionally, a little bit of Red goes a LONG way - enough so that there was a bit of experimentation about squeezing the Red out of a gem in order to make the other elements stronger - and you should be able to get by without boosting the Red in the gem at all. Pure Yellow Amplifier gems will increase the Tower gem's killing power against the targets it does hit.
As for what to bomb to... there is an unstated statistic that I refer to as a gem's Killing Power. That is found by taking the gem's maximum damage, and multiplying it by the gem's critical hit multiplier. You have a similar idea with your formula, but I go by max damage, not min damage, and I figure that enough hits are going to be fired that the 80% critical hit chance is almost guaranteed (if this hit doesn't crit, the next two or three should). Just bomb monster waves until the next bomb would exceed that Killing Power.
That said ... you need to be VERY careful. Bombing with lower Grade gems (Grades 1, 2, and 3 - not sure how much higher this applies to) boosts monster Armor tremendously compared to their HP. Armor is a straight reduction of damage (do 1100 damage hit to a monster with 950 Armor, and you only actually land 150 of that damage) and if you bomb a monster wave too much with very low Grade gems, you can easily have Armor you cannot scratch when the monster HP reaches your target level.
With that in mind, if the gem you are bombing with boosts Armor more than Hit Points, then you want to bring the wave's Hit Points + Armor up to without exceeding the gem's Killing Power. If you are bombing with higher Grade gems that boost monster Hit Points a lot more than the Armor, then just go by Hit Points.
Just remember, the higher the Grade gem you bomb a wave with, the more experience you'll get killing each monster. If you have the Mana, and have yet to bomb the wave to your gem's Killing Power, it is usually better to bomb with the higher Grade gem.
Now, you say you are not comfortable setting up a Mana Farm. That's fine. Here's some advice on getting that set up.
Set up your Killing Trap area first. Six amps (use Walls as placeholders if you need to) around a central Trap, so that they all boost the trap. Then place two more traps right next to the first one. This gives you a 3x3 area with three traps and 6 amps. It can be straight or curved :
[A][A][A] . . . . . [A][T][A]
[T][T][T] . . . . . .[A][T][T]
[A][A][A] . . . . . [A][A][A]
Then set up the manafarm area. Set your central trap, then its Amplifiers (again, if you cannot afford all the Amplifiers, use Walls as placeholders for them). The mana farm area should follow the same setup consideration as the killing area; you'll want to be able to prepare 6 amps all boosting a single trap, and you'll want a second trap on the inlet side (unlike the killing area, you do not need a trap on the outlet side). (The manafarm area should be an area that all the monsters have to pass through BEFORE they reach the killing area.)
Take your remaining mana and start making gems. Make your killing gem. 1 Red, some Black (or White), and some Yellow. Make your mana gem (1 Red, some Black or White, and some Orange). Place those gems into their traps. Make some pure yellow gems for whatever amplifiers you have at the killing area. I'd suggest you learn psorek's 32 spec gem recipes, and his 64 gem upgrade plan, but those can honestly wait until you are higher Wizard Level and have more starting mana (and higher skill caps). These simple gems will work for low level farming.
Find your killing gem's Killing Power. G1 Bomb the first wave so that its Hit Points + Armor come close to, without exceeding, that Killing Power. After the gems have all set up, find the killing gem's amplified Killing Power. Bomb the next few waves (again, Armor + Hit Points) to that Killing Power. When the spell refreshes, hit the killing gem with Bolt. At that point, you can ignore the monsters' Armor and bomb so their Hit Points come close to without exceeding the gem's Killing Power.
As you gain Mana, complete the two areas. As you finish the killing area, figure out the Killing Power and bomb to it. After you've got the two areas set up, it is time to use the other three traps I told you to build. Take a Grade 2 White gem, a Grade 1 Red gem, and a Grade 1 Dark Blue gem. Have the G2 eat the other two gems, and then duplicate the result twice. Put those into the traps, and (if necessary) use U to bring them up to 90% slow.
Every so often, upgrade your managem, killgem, and their amps. I personally find the Mana Lock Battle Trait useful as a timer.
If you have trouble getting this to work, it may be your Skills. You want your Skills (those that you've put any points into) at a multiple of 3. If you need more mana to get started, try reducing a few skills by 3 levels. If you can create both areas, fill them with gems, and still have a ton of mana, try increasing skills by 3 levels.