As I mentioned in the comments, you're looking at the wrong aspect of armor. You shouldn't be looking at the percentage it reduces, but rather the amount of damage it actually prevents as a measure of how much longer that armor lets your survive.
Let's consider an example for a champion with 0 armor. Say he has 1000 health, just 'cause that's a nice round number. Now our champion gets caught in Garen's judgement, and starts taking 100 physical damage each second (at one hit per second). Clearly, our protagonist can take ten hits before dying.
But now, imagine our champion has the 50 armor you mention, meaning the incoming physical damage is reduced. Now (still with 1000 health) it takes 15 seconds for that bastard Garen to claim our champion's life. 50 armor extended our life by 5 seconds. (100 * (2/3) = 66.6 DPS; 1000 / 66.66... = 15 hits)
So our champion respawns, wises up, and buys 50 more armor. Now with 100 armor, we're sitting at a flat 50% damage reduction. Again we run into Garen, who's still dealing 100 base damage per second. It now takes him twice as long to kill us -- 20 seconds. Once again the armor has extended our life, again by 5 seconds. (100 * .5 = 50 DPS; 1000 / 50 = 20 hits)
Back in base, we buy another 50 armor. We're now at 150 armor, and 60% damage reduction. Enter Garen -- spinspinspinspinspinspin and it takes 25 seconds before our champion dies. Again, adding 50 armor extended our life by 5 seconds. (100 * .4 = 40 DPS; 1000 / 40 = 25 hits)
So even while the amount of damage reduced per point of armor diminishes (as you observed), the amount of time armor extends your life (or effective health) remains distinctly linear.
I don't think this is possible in LoL. Anyways, you will certainly not be able to delay them long enough to be significantly closer to your own tower. However, you can easily let the minions fall back to your own tower, by not attacking the enemy minions (except for last hits).
As long as your lane opponents attack your minions, while you only last hit theirs, you will be at your own tower within one or two minion waves.
The worst possible case is if you're solo against two enemies. Then they will position themselves between you and your minions to drive you away, hence, you will not get any experience. In that case, there's not much you can do (if you engage at level 1, you normally die). However, the opponents will last-hit your minions, hence, pushing theirs. Again, they can only starve your XP for a wave or two, before everything is taking place in your tower's range.
(Note, however, that you usually don't want to fight in your tower's range all the time against ranged heroes. They will continually be able to hit your tower and eventually get it down.)
Best Answer
You won't be able to dodge the projectile by flashing over it. Once the shot is fired there is no way of dodging it by movement alone.
The only ways of dodging a Cait Ult are: Spellshields, stasis or someone standing between the initial target and the projectile.
Flashing behind the ult will merely cause it to change directions. The same happens if you are being teleported somewhere else.