A powermeter is all well and good, but your "internal pacer" is just as important. For me, the numbers are just there to show how good or [censored] I am on that day -- the perceived effort is the same whether I'm on a PB time for a 10 mile TT or 10% below what I should be able to do.
It's handy to have the data though, as you can use it in the early minutes of a TT to limit your efforts (not go out too hard), in the middle to keep pushing, and in the final minutes to eke out every last watt. Also useful if your TT has a headwind and tailwind section, or is rolling/hilly then flat, or whatever -- you can use it to get the most out of your effort depending on your own strengths and weaknesses.
For road racing, it's mostly on feel too for me -- again, my watts just show if I'm doing well, average, or badly. My best result last season in a big road race came where I bridged to the break for ~5 min on nothing more than threshold watts (all I could do!) but I kept going because the bunch let me go and I could see the break in sight and just needed to get to them before they crested the climb and went downhill (I'm a climber). From a numbers perspective, nothing special for me, but had I sat up once I saw I had nothing better than threshold watts to get across to the break, I would have missed out on a hard-earned 4th place in that race. Other times I've instigated a solo break early in a crit race and realised from the numbers that my legs were great and that just gave me more incentive to keep going and win.
YMMV but remember that powermeters and data are descriptive not prescriptive. You don't start/drop out of a break because the numbers are good/bad. You do it based on feel and what's going on in the race, and what the other riders do, same as you would if you had no powermeter. The numbers just help tell the story afterwards.
If you take my 60min/25mile TT power as 100%, then my best ~20min/10mile TT power has ranged from 105% to 112%. My best 10min/4mile TT was 122%. Those were all flat high-speed TTs though; uphill I can add another ~5% to all my values.