The best way is to look at past finishers times and splits. For your given target time, find 5 (more if you want) finishers with that time. Then take the average of their times for each split. You can get a very good read this way.
The hardest part will be estimating what a good (realistic, yet still stretching you) target is for your finish time.
This post has some estimates of how to do this based on your FTP.