You're right that using flashcards for quantitative chemistry is pointless, since those questions test you on the process to get to a certain answer rather than straightforward facts. I'd recommend you do past paper questions, because doing enough of them will retain the knowledge in your head. It's like maths, once you do it enough, it becomes routine and you'll be able to do it without even thinking too much about it. Don't worry about needing to do too much past papers, an hour or two (or maybe a few depending on your ability) will definitely be enough for you to get it in your head. Good luck!