Trevor Dupuy considered surprise to be one of the most important human factors on the battlefield.
A military force that is surprised is severely disrupted, and its fighting capability is severely degraded. Surprise is usually achieved by the side that has the initiative, and that is attacking. However, it can be achieved by a defending force. The most common example of defensive surprise is the ambush.
Perhaps the best example of surprise achieved by a defender was that which Hannibal gained over the Romans at the Battle of Cannae, 216 BC, in which the Romans were surprised by the unexpected defensive maneuver of the Carthaginians. This permitted the outnumbered force, aided by the multiplying effect of surprise, to achieve a double envelopment of their numerically stronger force.
It has been hypothesized, and the hypothesis rather conclusively substantiated, that surprise can be quantified in terms of the enhanced mobility (quantifiable) which surprise provides to the surprising force, by the reduced vulnerability (quantifiable) of the surpriser, and the increased vulnerability (quantifiable) of the side that is surprised.
I have written in detail previously about Dupuy’s treatment of surprise. He cited it as one of his timeless verities of combat.  As one of the most powerful force multipliers available in battle, he calculated that achieving complete surprise could more than double the combat power of a force.
Did the envelopment itself have a defined “modifier range” as well?
He did not attribute maneuver specifically to achieving surprise, though he undoubtedly would have agreed that it could.
Interestingly, maneuver did directly contribute to the concept of involuntary changes in combat posture, i.e. breakpoints. Envelopment, encirclement, or penetration of position are the most common reasons cited for a unit to retreat involuntarily in combat. See Chris’s post: https://dupuyinstitute.dreamhosters.com/2017/06/23/battle-outcomes-casualty-rates-as-a-measure-of-defeat/