[ARC5] Smart People: Attenuators

[David Stinson]

Need the help of you smart people again.
I built a "utility" linear HF amplifier to help put QRP transmitters
on the air.  Works great with the correct drive,
which is 10 milliwatts.
I need to attenuate drive levels from a couple of hundred milliwatts
 up to about 6-7 watts, so need a variety of pi-type attenuators.
The resistance values are no problem,
 because someone wrote a Java app:

http://chemandy.com/calculators/matching-pi-attenuator-calculator.htm

My question:
What's the formula for the Wattage rating of each resistor?
For instance:  If I have a 4-watt drive signal and need to get it down
to 10 milliwatts (assuming 50 Ohms in and out),
I need to attenuate 36 dB.
That's shunt in 52 Ohms, series 1580 Ohms, shunt out 52 Ohms.

How to calculate the appropriate wattage of the three resistors?
Common sense says the shunt input should be big enough to
sink the entire carrier power, while the series and shunt out
could be smaller.  What's "right?"