I generally find aqadvisor to be a handy tool when I'm planning a tank, and their values, numbers, and compatibility advice are generally on the right track. If you stay under their recommended 100%, you generally won't be overstocked.
However, I'll share the same advice others have: As useful as this site is, it is only a starting point, and far from perfect. Research the different species you plan to keep, including preferred hardness and numbers. seriouslyfish.com is a go-to site for research. Planetcatfish, scotcat, and loachesonline are also generally quite good. And of course, you can usually find someone on this site with real-world experience with whatever you're hoping to keep.