Only ever as a “probability function”; never with certainty. You study and analyse and monitor, and recognise “set-ups” that “quite often represent reversals” - a significant enough proportion of the time for trading them to be likely to produce more profit, collectively, from the winners than losses from the losers (of which there will always be some).
Some basic understanding of probability and statistics is absolutely essential. (Recommended book: [I]Profitability & Systematic Trading[/I], by Michael Harris - Wiley 2007).
Key concept: you work out your position-size by starting off by knowing the appropriate size of stop-loss for your trade (according to the volatility, or with whatever other method(s) you use to determine that), and then from that working out what size of trade will represent 1% of your account, [B][U]not the other way round[/U][/B].
Again, this needs to be determined according to your trading methods/system. Not a very helpful answer, perhaps, but the reality. You set your TP wherever your research has shown that TP’s in that market, for that instrument, for trades of that type, have a [I]net positive expectancy[/I]. You’ll [I][U]never[/U][/I] know confidently that any [I]individual[/I] trade will produce a profit (and nor will anyone else). What matters is to know confidently that the next 100/200/300 trades of that type, on that instrument, with that SL and that TP, will produce a [B]collective[/B] profit.
On the subject of position-sizing, specifically, the first page of this thread will probably help you, if you read it carefully: 301 Moved Permanently.