fudge factor

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun A variable factor or component used in calculations or experiments that allows for a margin of error or produces a desired result.

from the GNU version of the Collaborative International Dictionary of English.

• noun a term or factor inserted into a calculation to compensate for anticipated errors, or to arbitrarily make the result conform to some desired conclusion.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun idiomatic, engineering A quantity introduced to compensate for uncertainty.
• noun idiomatic any padding or compensation built into a guess or estimate

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• noun a quantity that is added or subtracted in order to increase the accuracy of a scientific measure