That's weird - 650/30? You're never supposed to take more than 1g of APAP/paracetamol per 4 hours, so the cocktail's usually 500/30 in European codeine/apap cocktails.
CWE'ing is easy enough - but impatience or a bad filter can let too much APAP through. Either google around for CWE instructions, filter the solution twice and use very little water - 50 mls should be enough - and don't use Melita brand filters, as they're full of holes, or keep the pills for hangovers.
But frankly, if you're opiate naive, I'd start with 120, just in case you're extra-sensitive: or better yet, save the pills for killing pain (which we can rely on to occur) and, if you are going to try opiates recreationally, wait until some better pharms come along. Even with promethazine, codeine is very short-acting and depending on how practically/technically minded you are, it can take some practice to perform a CWE. Without opiate tolerance, you have a one-pill hangover cure that you could take with a little ibuprofen (or an ibuprofen/codeine cocktail, if they're OTC in Germany, as in much of the EU) and rebound from a big night mixing beers and cocktails or whatever.
Low-dose opiates, without tolerance, are much more useful for legitimate painkilling use than recreational, and if you want to experiment, my German friends never seem to have trouble finding opiates, from H (not a good place to start, either) to oral morphine and DHC. Dihydrocodeine is much, much more pleasant than codeine simple, and I think quite common in Germany and Austria, also, there's no ceiling effect, so you can try 30mgs (the usual low-dose prescription), give it 90 minutes, take another 30, etc, if you want to/till you're where you want to be). Keep the pills you have, and if you're going to try opiates, I'd say get them in a pure form, rather than starting into CWE's. They're always a little risky - and just not worth it for 240mgs of codeine. Still, if you're intent on trying, filter carefully, and like other posters have said, you need a sedating anti-histamine: either diphenhydramine/doxalymine succinate (former usually sold, in Europe, as an OTC sedative) taken 60-90 minutes BEFORE the codeine, or, ideally, Promethazine (Phenergan/Sominex in the UK at least), 30-45 minutes AFTER the codeine. Otherwise you may scratch till you bleed - codeine's not great at letting the endorphins fly, but it does give you one well of a histamine rush, and any of the above anti-histamines improve the codeine experience, whether through synergy or potentiation.
Sorry for the long answer to a simple question, but I'm prescribed 500/30s for migraines, and although I now have a pretty high opiate tolerance, it all started with CWE'ing codeine: which led on to harder opiates, and also left me without painkillers when I actually needed them. If you're remotely prone to addiction/binge-drinking, then it's probably a good idea to stay away from opiate highs and use them only in circumstances of legitimate medical need. A lot of people on BL, if we could go back in time, would NOT take that first taste again - whether codeine or H or vicodin or whatever, even a little can be the first step on a long, long journey downwards. I'm not moralising - I still use, and probably always will, occasionally, but I lost a good few years to addiction, and it all began with kratom and codeine.
Either way, play safe and enjoy.
You cannot add more codeine later as it inhibits itself.
