text: Perhaps that is more an art than a science.  Is the project on schedule?  What % of the budget has been expended?  What % of the materials have been used?  What % of the estimated construction time has elapsed?  What % of the project footprint has been completed?  Can you break down the project into pieces (square feet of floor?) and estimate % completion of the pieces then sum up the pieces?  For example, three rooms (1/3, 2/3 and 3/3's done for 6 parts of 9 parts, etc.  What can be excluded from your calculations; 70% complete except for landscaping?  Importantly, who are you making the completion estimate for and what is their major concern (budget? time?).  Good luck!