The Low Power FM (LPFM) radio service, which was created by the Commission in January 2000. 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 a 100 watt LPFM station 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.
LPFM radio stations are available to noncommercial educational entities and public safety and transportation organizations, but are not available to individuals or for commercial operations. Current broadcast licensees with interests in other media (broadcast or newspapers) are not eligible to obtain LPFM stations.
LPFM radio stations must protect authorized radio broadcast stations on the same channel or frequency (cochannel), as well as broadcast stations on first, second, or third-adjacent channels above or below the LPFM station’s frequency. This protection is accomplished through the use of minimum distance separation requirements, which are set forth in 47 CFR 73.807
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I live in an area small town no real tall building and the terrain is pretty flat. The town is surrounded by farm land. I Believe a 30 meter HAAT tower with an ERP of 100 watts could go much farther than 3.5 miles. Maybe 8-10 miles. Does the FCC require a LPFM station to turn down till they are only reaching 3.5 miles or can you stay at full power?
The 3.5 miles refers to the city grad 60 Dbu contour. This is a measurement of signal strength at the given distance. The signal continues on with decreased signal strength, ie 50 Dbu, 40 Dbu. etc. On a clear frequency the LPFM might be heard 25 miles depending on terrain and interference level. After you receive permission to build and build the station, an engineer would determine the proper power level of your transmitter. This would depend on the type of antenna,coax type and length, height and average terrain. Once this has been calculated the TPO cannot be changed unless you change something in the system, ie different antenna, longer, shorter or different coax type. The 60 DBU contour is considered city grade as anything within this contour should be very strong with ability to penetrate buildings.