The TV weather reports are using the dew points more than the relative humidity to indicate the discomfort level as being more representative of what most of us feel, although the discomfort level varies by location (Miami, where they are used to high humidity, versus Las Vegas, with very low humidity), and even from person-to-person (my wife versus myself).
But, for Philadelphia, in the summer months, the dew point is the best measure of discomfort for most of us, and is:
Dew Point (ºF) Comfort Level
61-65 Getting sticky, especially at night
Relative humidity is determined by a ratio of vapor pressures that can be misleading, so says the Federal weather bureau web site, “A temperature of 30º and a dew point of 30º will be a relative humidity of 100%, but a temperature of 80F and a dew point of 60F [calculates as] a relative humidity of 50%.”
But keep in mind, this is only a subjective guide for most of us in Philadelphia, in the summer. You, in a different location, in a different season, may feel differently.
Therefore, bottom line, easy to remember, you want the dew point to be below 60ºF.