"optimum flight hours" is keeping the aircraft in the air except for mandatory maintenance.
In the 1940-1950 era a DC3 was considered to have a "good" route if it was in the air 2-3 out of 24hrs.
Today, airlines keep aircraft out of the gate closer to 16 out of 24hrs.
Total Airline Operating Cost Breakdown
======================================
44% is operating, includes fuel, direct maintenance, depreciation, and crew
29% is servicing expense, aircraft, traffic, passenger
14% is reservations and sales expense
13% is overhead expense, adds and admin
Cost per block-hour of operations
==================================
B757-200 (avg. 186 seats)
Based on 1252 mile average stage length and 11.3 block-hr daily utilization
CREW $ 489
FUEL $ 548
MAINTENANCE $ 590
OWNERSHIP $ 923
--------------------------
TOTAL $ 2,550 per block-hr
Typical costs per block hr
A/C Seats blk/Hr seat/hr
===================================
DC9-30 100 $1,973 $19.73
A320 148 $2,270 $15.33
B727-200 150 $2,555 $17.03
B757-200 186 $2,550 $13.71
B747-400 375 $6,455 $17.21
Source