As you probably know, dead-down-wind (DDW) is not the fastest point of sail; you're fastest speed downwind and optimal VMG may be 15 degrees off of DDW (as an example). By knowing where you are relative to the fastest point of sail downwind, you'll know when you should gybe.
Ideally, you're sailing downwind 15 degrees off of the wind. If the wind or desired direction puts you closer to DDW, then gybe over and sail 15 degrees off the wind on the other tack. Staying as close to the 15 degrees, and knowing when to gybe over to the other tack, keeps you near, hopefully, optimum VMG. Any wind compass can tell you approximately; for a grand+, Tacktick may claim to do it better than others.