Well, you could run gobs of speed tests on different machines to cover allt he different situations...

But of course, there are ways to mathematically describe precisely what you are doing. From these equations you can then gain a better understanding of how things work, and develope heuristics. Unfortunately I left my database design books at school, which is a few hundred miles away. I forget who wrote the first article, prolly someone like Jim Gray. Anyway, you might not have encountered them too much in design, because they come in handy moreso if you are designing a db (e.g. oracle, vfp, mysql, etc.) but they are real handy to know anyway, because like I said they give you a better understanding of the implications of the choices you make.

matt