Unfortunately, you will need to know MP and RPM to ascertain power, unless the manufacturer has calculated a table of RPM and altitude for power for your particular engine/prop combination.
If you are running at peak EGT or leaner then fuel flow correlates closely with power at about 0.4 lbs/hp/hr. E.g 8 galls/hr = 48 lbs/hr = 120HP which would be 75% for an O320 or 67% for an O360. If you're rich of peak that would be an over-estimate.