I42, Gee in the Varsity was a rapid fixing aid! Having installed the correct box for the chain in use you would fiddle around with the oscilloscope and using a n electronic upscale measure the time differencees .Having logged the readings you plotted them on the hyperbolic chart.
Properly you should now log the position on the hyperbolic chart before transferring the position to the plotting chart. Having now established your position, and considering previous positions you could work out the groundspeed. Using that and the position you could work out the ETA.
Quite simple really. You could also use the AP I to calculate a wind and then groundspeed.