Converting cumin seeds to ground cumin
More important than exact measurements: Are you toasting the cumin before using it? Only toasting developes the true whole cumin flavor.
Easy to do: in a dry heavy skillet, over medium heat toast the whole seeds till they give off an aroma (stirring or shaking the pan often). Cool and grind in a seed grinder.
Commercial ground cumin is more likely to be oxidized (stale) but can be toasted for optimum flavor as well. Just watch it so it doesn't burn.