Ok, interest piqued so had to look up some figs for this.
Radar safe distance is Max Permitted Exposure Level (MPEL). MPEL is a function of antenna/dish diameter, rated peak power output and duty cycle (time from start of one pulse to start of next pulse transmitted) - radeng explains a lot of this.
The American National Standards Institute (ANSI) gives an MPEL of 0.005 Watts per square centimetre though other national standards of 10mW/cm squared are sometimes used.
Old weather radar sets had to transmit high power because the receiving processor electronics were noisey and needed a higher power return to be able to show a picture of the weather. This isn't the case anymore. Some of the power outputs quoted above would do an Air Intercept radar on an F15 proud.
With newer, more sensitive processing electronics, a good return for showing weather can be of a much lower strength so the transmitter power can be a lot lower.
Transmitted power outputs on modern weather radars range from 1,300 watts (1.3kW) to 10kW giving Radar Safe Distances of 2.7ft for the Bendix RDS-86 fitted to the BAe ATP to 13ft for the Honeywell RDR-4B on large Boeings and Airbus.
So to answer the original post's question - the plane in front was quite safe.
Rule is though, don't switch the thing to transmit until you are taxiing out. Some aircraft have system disables installed but the pilot is the best controller of all.
Sorry you asked now?