EE question about duty cycle

[M&D]

EE help needed:
It seems that I have some extra board space on the
O2 meter I'm working on... and was wondering how
to implement injector duty cycle measuring. I can
figure out the PRF measuring easy enough, but how
to combine that and PW to get a percentage I'm
lost. Help!

TIA

PS -it looks like I can get the boards relatively
cheaply (~$20 for the 2 small boards the design is
composed of) so I'll let everyone know when it's
implemented.

Michael Harrington
91coupe at bellsouth.net and
http://www.geocities.com/motorcity/downs/4038
91 RX-7 Coupe "Pearl" suspension blues
88 RX-7 T2 "Beast" Alive again!
N3WJE Technician+ Ham