The following was taken from the FCC Website:
These stations are authorized for noncommercial educational broadcasting only (no commercial operation) and operate with an effective radiated power (ERP) of 100 watts (0.1 kilowatts) or less, with maximum facilities of 100 watts ERP at 30 meters (100 feet) antenna height above average terrain (HAAT). The approximate service range of these stations is 5.6 kilometers (3.5 miles radius). LPFM stations are not protected from interference that may be received from other classes of FM stations. A construction permit is required before a LPFM station can be constructed or operated.
This and more information can be found at www.fcc.gov.
There are really too many factors to put into an equation that figures distance from your transmitter and antenna. I have heard of ERP's as low as 1 Watt up to 100 Watts, it really depends on the geography around the transmitter site.
Give me a call at 800-446-2295 to talk about more specific information. Ask for Brian in the Sales Dept.
Hope this helps.