Performing a 'reduced thrust' takeoff properly is not a problem at all. In fact, in the instance that there may be an engine failure after V1 and a reduced thrust takeoff was used...may help ensure that you have a good remaining engine(s) when you really need it.
The takeoff performance is a simple calculation using a runway analysis chart for the airport. To keep from creating an error, and a good CRM technique...both pilots do the calculation separately and compare the results...if the Assumed Temp. and numbers are the same, then away you go...if not...both back in to the books again.
Bear in mind, there are several reasons that reduced thrust takeoffs cannot be made..wet/contaminated runways, special airport procedures, or MEL/CDL limitations to name a few.