Set height & heading hold, then just nudge the heading bug or autothrottle datum every so often (maybe every 3 mins or so). The F3 Tornado used less fuel if you used manual throttle.
Not in true close formation though. Fly a tight battle; quick radar scan ("just one ping, Vassily") to check lead's speed.
...and take a mini height split (say 80') in case you fall asleep.