I like to use something known as the BU:GU ratio. That is, bittering units (IBUs) to gravity units (the last three digits of original gravity). So, for examples:
1.060 beer with 60 IBUs, BU:GU = 60/60 = 1.00
1.060 beer with 30 IBUs, BU:GU = 30/60 = 0.50
1.060 beer with 15 IBUs, BU:GU = 15/60 = 0.25
As a general guideline, IPAs are around a BU:GU ratio of 1.00, a balanced beer is around 0.50, and a beer that is very malty and not so hoppy can be anywhere around 0.10 to 0.40. So, pick what kind of balance you want, then use your original gravity to determine how much hop bitterness to add. It's not an exact science, but it can get you very close to where you want the balance to be.