If you're doing tray development, get a dark green safe light, and develop by inspection. It's really easy. Just develop for 20% less than you think it'll take, turn the light on for a few seconds, look at the neg, throw it back in the developer if it needs more, repeat every minute or two until development is finished. Voila. You have a ballpark figure for how long it takes.
It has been tremendously helpful to me, I've never had to ask for developing times since I did. I do this with sheets, and then develop rolls roughly according to that.