Power density and dBm units

A document says the spectral level (in Watts/Hz) of a pulse with pulse repetition frequency f (in Hz) and pulse duration t (in seconds) and pulse peak power P (in Watts) is given as below.

spectral_level (Watts/Hz) = f.P.t.t 

Then, the article converts the spectral level from Watts/Hz, given above, to dBm/kHz (that is dBm per kilo Hertz) as below where F is f converted to kHz, T is t converted to microseconds, and P(dBm) is peak power converted to dBm. I know that dBm = 10log(power in milliwatts / 1 milliwatt), but I’m not sure how the term -60 is added in this equation.

spectral_level(dBm/kHz) = P(dBm) + 10log(F) + 20log(T) - 60