The units of MIL-HDBK-217 are in operating hours. Flight hours would be a subset of operating hours. One approach to convert the units to Flight Hours would be to determine the ratio of flight hours to operating hours, then multiply the MIL-HDBK-217 result by that ratio. You should understand, however, that in reality, particularly with a severe environment of ARW, the vast majority of failures may occur during actual flight. Using the ratio technique, then, may result in an unrealistically optimistic reliability prediction compared to actual field experience.