Have seen different date in differenet sourse for ACCURACY of ESTIMATES

Rough order of magnitude estimate ROM: this type of estimate is made during initiating process. Typical range is +/-50 percent from actual. in other place, have seen -25% to +75%...

Budget estimate: this type of estimate is made during Planning phase. Typical range is -10 to +25 percent from actual.

Definitive estimate: Later during the project estimates will become more refined, some PMs use the range of -5 to +10 or -/+10.