My DX-7 has a timer on it. From what I've heard, the only problem with the timer is, it depends on what type of throttle you got. If you run full power all the time, your gonna have less running time. You said your going about 3/4 throttle, soI'm not sure how to set the timer exactly, but maybe fly it so many minutes, land, check fuel level, and keep doing that till your very close to empty. That may have sounded confusing, but thats how I did it on my AT-6 texan.