Here is a nice on-line application from VOA. It only goes down to 1 watt but you can select the location lat/long and antenna. Uses current progation data.
http://www.voacap.com/coverage.html
Have fun!
So I modeled my location @ 1 watt, selected the 20 meter band, 1/4 wave vertical and AM as the mode. Based on today's propagation and time of day, I would have a 20 to 30% chance of being heard in the Caribean and North Georgia and South Carolina. Now keep in mind that 1 watt is over the limit but does give you an idea of what is possible if the FCC would open this or 6.9 to 7 mHz to experimenters. Also at 13 mHz the ground wave locally would be nil.
I've used this tool a few times. It is pretty cool to see what power levels and different modes are capable of.
Very Useful Information.