does it matter so much that it overestimates? It's really tedious reading all these posts knocking the Taxc power meter as if there are guys claiming to be world class when in fact the device is overestimating by 10% or whatever (that's a cue for some pedantic to point out that it's probably more like 20% or whatever).
The point is that if you calibrate the thing correctly each time you can see where you're at, see how you progress and set yourself targets. In the long term you could try and see if you can add 50w to your threshold over 3 months and in the short term you can use the meter to force yourself to stay above 450w for 5 minutes or something similar. As long as you're using the trainer to push yourself and seeing improvements it's a good thing. Accuracy would only matter if you're insecure about the level you're at and need the security of knowing you can (or could if you tried) hack it in your category....or if you're the "racing against yourself" type who needs to know exactly how you compare to a professional....
I've said it before but I'll say it again - by all means use the tools to help training - but the truest test of cycling ability is what level you race at and how you're doing. Train hard to better these.
