There are two ways I can think of (both are using features on the Sekonic L358, as that is what I have and can't really comment on other light meters) there is one setting where you set it to wait for the flash to go off then meter, so you need someone to hold the lightmeter/fire the camera & flash for you.
The second method costs a bit more, you need pocket wizards, a pockt wizard chip for the L358 and a motordrive cable for you camera. Unless your D60 has a PC port for firing non hotshoe flashes you will only beable to use the built in flash as you can't mount a PW on the hotshoe with the flash up. Essentially you use the L358 to fire the camera attached to the PW which causes the flash to flash and when it flashes the metering happens.
That probably doesn't make much sense, but hopefully you get the idea