At the current difficulty, you have a 0.001239% (rounded) probability of finding a block within 10 minutes at 300 MH/s. So basically you're wasting time that you could have spend contributing to the pool and getting a higher split. Then if you beat the pool and find the block solo, you get 50 BTC but lose all the progress in the current pool. And that's like betting on red and black at the roulette table except if the payments were less than your bet. One is going to lose and you're betting your mining time on opposites. If you thought you could beat the pool solo, you should solo the entire time.
The same goes for mining until 43.5% of what I assume is the estimated completion time of a block since there is no "progress" or "completion percentage." You bet on two pools, only one can win, you're losing your mining time on whatever pool loses. It doesn't seem logical.
Tell me this. Let's say the proportional pool has (2*difficulty) shares in the current round. If you mine for it now, then no matter what happens, you will get less than (Block reward)/(2*difficulty) per share you submit. So why do it instead of, say, a PPS pool which gives you (Block reward)/(1.1*difficulty) per share you submit?