We did that at my old shop. If the book time labor would be 1 hour, we'd charge 1 hour labor. Usually it took whoever was doing it much less than 1 hour. That gave us some room to negotiate if the customer didn't like the price, but it is 100% legit. On the other hand, sometimes book labor would be 1 hour and it'd take someone 2. We just ate the extra hour labor at that point. What's that saying "Some times you get the bar, some times the bar gets you"