By Barry W. Boehm, Prentice-Hall, Inc., 1981, 0-13-822122-7 The classic text on waterfall development. You need to read it to understand the “norm”. Barry Boehm clearly has a long experience with large teams.
[p39] If the software requirements error is detected and corrected during the plans and requirements phase, its correction is a relatively simple matter of updating the requirements specification. If the same error is not [p40] corrected until the maintenance phase, the correction involves a much larger inventory of specifications, code, user and maintenance manuals, and training material.
[p310] When we first begin to evaluate alternative concepts for a new software application, the relative range of our software cost estimates is roughly a factor of four on either the high or low side.
These ranges have been determined subjectively, and are intended to represent 80% confidence limits, that is, "within a factor of four on either side, 80% of the time."
[p311] By the time we complete and validate a product design specification, we will have resolve such issues as the internal data structure of the software product and specific techniques for handling the buffers between the terminal microprocessor and the central processors on one side, and between the microprocessor and the display driver on the other. At this point, are software estimate should be accurate to within a factor of 1.25, the discrepancies in caused by summer main sources of uncertainty such as a specific job earnings to be used first-half scheduling, air handling, abort processing, and the like.
Within the factor of 1.25 range (80-125%), a good software manager can generally turn us offer effort estimate into a self-fulfilling prophecy.