Calculate Your Miles Per Watt

 First determine your QTH locator with the excellent: Find your

QTH locator by F6FVY Laurent Haas.
Zoom in on your city and then click on your street to find your QTH locator.
Then look up the QTH locator of the station you worked.

Then calculate the distance with the fantastic "Miles per Watt calculator" by N9SSA. 

Copy the two QTH locators and paste them, one by one, into the fields ”Grid-Square”. You can your contacts grid square from the QRZ.com website. under "detail"

Fill in the power in the field ”Watts”. (give 0.05 for 50mW)

Press the button ”Calculate Distance” and be surprised. (hi)

My QTH grid square is QH31ar

Here's my quick reference guide that I can use at a glance
Make a QSO using 5 watts or less power and a simple wire antenna in which distance(miles) / power(watts) is greater than 1000.

1/4 Watt (250mW)

Everything outside the YELLOW circle is 1000 MPW

1/2 watt (500mW) 

Everything outside the RED circle is 1000 MPW

1 watt

Everything outside the GREEN circle is 1000 MPW 

2.5 watts

Everything outside the BLUE circle is 1000 MPW