I've used tuneecu but more recently a home made manometer (do a google search if interested - no need for a fancy vacuum gauge set on a twin)
Tuneecu got it pretty close but the values fluctuate so its hard to judge when they're averaging the same. I hooked up the manometer, (basically a tube with oil in it) to the tb vacuum ports and needed to make a slight adjustment, so I'd say that is more accurate.
I've removed the canister air pump and replaced with test leads, so the tb sync is easy to hook up, but if I hadn't done so I'd definitely get one of the gauges Geek suggests. It would be a ton of work to mess with vacuum hoses just to tb sync