We've been using an old TAG with O2 sensor. It's surprisingly accurate once it's built up a decent data base. If you a re constantly tweaking the combo like we do, it will skew the figures by giving more more of a "predicted deviation". We usually reset the data base if we have made a big change in the combo, we aren't proper bracket racers. We are usually chasing an ET's in a bracket class, as over here there aren't many heads up classes but it's the best way to run on a good track.