Resistor power rating for short pulses
Pulse handling is a highly variable characteristic. You can often find this this information in manufacturers' handbooks, occasionally on the 
datasheet. Some resistors are specially designed for pulse handling capability and that will be well characterized. Things like the trimming method can affect the maximums- I would suggest not taking chances if reliability is important. 

Here is such a resistor (Panasonic ERJ-P08J101V surge-resisting) which compares the pulse handling ability with more conventional resistors (note the test limits- 1,000 cycles and +/-5% permissible resistance change). 



In this case, a 2/3W resistor can withstand 20-40W for 1msec (depending on type) so your 10x for 1usec seems fairly conservative (at least for the type of resistor I looked at, and with the allowable number of pulses and resistance change).