Hi,
As you have historical data available ( 30% variation from the actual projection), you can review your earlier estimates and prepare a rough cut planning data of the resources used in manufacturing the finished product. This planning data you can use for your upcoming orders.
Also, project manager should allocate budget to project with some mark up value (15-20%) for avoiding any process delay in project execution due to budget overrun.
Above all, the process can be improved with a "Proper and Accurate Planning" of work objects (WBS) and its resources.
I am willing to know of what difficulties project engineers are facing in doing thorough planning of work packages (WBS,activities,resources etc.)?
Regards
Saurabh