Direct Answer
Track revision costs by tagging all labor and material that is specifically driven by a design change: detailing hours for model revision, shop hours for rework or new fabrication, scrapped material value, and any premium charges (expedited CNC programming, priority material procurement). Maintain these records by revision number, linked to the drawing or RFI that caused the change.
Full Explanation
A revision cost tracking system requires: (1) Timecards that include a 'reason code' allowing workers to tag their hours to a specific revision or change order number. (2) A material log tracking scrapped material by cause. (3) A detailing cost log showing revision hours by drawing number. (4) A summary report that rolls up all costs by change order number. Even a simple spreadsheet with these fields provides the documentation needed for change order negotiations. Many PM software platforms (Procore, Viewpoint) have built-in change order cost tracking modules.
What This Means for Your Shop
Change orders that are supported by detailed cost records are settled faster and for higher amounts than those submitted as lump-sum estimates. A GC who sees 'Labor: 42 hours × $85/hr = $3,570 (timecard records attached)' has much less room to negotiate than one who sees '$3,570 — trust me.'
Common Mistakes
Not starting cost tracking until you've already absorbed significant revision costs. The tracking system needs to be in place at project start, not when you realize you've lost $50,000 to revisions.
Recommended Process
Set up your revision cost tracking system at project kickoff. Train your shop foreman, detailer, and purchasing agent to tag all change-related time and material in the system. Review accumulated revision costs at every project meeting.