Accepting your calculations... obviously, if you're 100% dead certain sure the price will be over $52 on Dec 30th you'd like to stop your weekly contributions for any week where the price is above $46.80 (ignoring transaction costs) and buy the stock in the market. And you ought to mortgage your house and hock your wedding ring and scrounge up as much cash as you can and buy at the absolute bottom and get rich. But if you have such Nostradamus-like predictive ability about the market, why are you working? LOL, I was not trying to predict the future.
For my part, I figure the DESPP is free money over any reasonable period of time, so I put the DESPP max, $10,000/yr ($192/wk), into it, and, since I can't afford to save that much and in any case don't want to be any more over-invested in one company than I can help, pull out the shares as soon as I've got 100+ eligible for capital gains treatment. That's when there are gains that you want to be taxed at the capital gains rate. This year it may make sense to recognize an ordinary-income loss by selling everything in December --- I'll have to think about the implications of that. Anyway, I don't try to outguess the market much. It's too hard to come up with a better estimate, today, for the discounted future returns on your stock than today's stock price. I'd try to avoid selling into a panic, but if UPS drops into the 30's and stays there for a few months you really need to come to terms with the fact that the stock you bought for twice that is really only worth half what you paid for it.