Most HOAs determine reserve needs by funding a periodic reserve study. The amount of appropriate reserves will vary widely by what services the HOA offers, and how much responsibility they take for structural issues.
An HOA for a condo complex that is responsible for deck safety, parking areas, structural walls, pools, and roofs will have much higher reserve needs than an HOA covering detached single family homes where virtually no services are provided. This first HOA must collect enough to fund the replacement of each of these items by the end of its useful life-- so a new roof every 20-30 years, new decks every 15-20 years, new asphalt parking every 10ish years, etc.
If you really want to see how well funded the reserve is, what you actually want to be looking at is the reserve study.