Look ahead (GI) adjusted machining time estimation

(Not sure if this is the right section)

I work at a job shop where we have 5 CNC milling machines. We do a lot of varied work and very little production work, hence it is very important for us to be able to predict the machining time within 5-10% accuracy, as it has a lot of impact on scheduling as well as quote calculation.

The machining time is largely off from the CAM software's predictions mainly because of the feed variations introduced by the controller's look ahead (Geometrical intelligence) function.

So I want to know if there is any way of deriving a more realistic estimation of machining time? Do the controllers have in-built simulation which i can use to get the timing data before actually running the program? After all its not like the controller varies feeds based on any feedback signals, if I'm not wrong.

Buy better CAM software.