Even if transfection manufacturers give a range of densities as recommendations, it's always good to determine the optimal density empirically. And to keep it constant once made.
HEK cells are easy to transfect and largely documented. As an example, to properly answer your question, with our Viromer RED, a density of 4.6 10^5 in 6-well gives the best results in our hands (routinely used for testing our reagent).
You can take a look to the manual of Viromer RED, on pages 4 and 12, you have tips for adjusting cell density. It works for any other reagent.