I train a lot at 100rpm (since it was in my training guide)

What I did was make a list:

100 rpm at 39x17 is about 28km/h
100 rpm at 39x16 is about 31km/h
100 rpm at 39x15 is about 33km/h
etc....

So I know by my current speed and gear if I am doing the (roughly) correct rpm. If not then I either adjust the pace or change gear.