Maximum putter face angle deviation from squared to the putting line for a straight putt can be calculated as the ArcTangent of the radius of the golf hole divided by the length of the straight putt:

Face Angle Deviation from Square in Degrees = ARCTAN(radius of golf hole/length of putt)

The radius of the golf hole can be approximated as 2 inches.

For a 10 feet (120 inches) straight putt, the maximum theoretical putter face angle error (deviation) from being perfectly squared at the moment of impact is:

arctan(2/120) = arctan(0.0167)= 0.96 degrees

The following table lists allowable deviation of putter face angles for straight putts for putting lengths from 5 feet to 30 feet:

Table of Maximum Putter Face Angle Deviation from Square for Medium Length Straight Putts on Flat Surfaces:

5 feet: 2 degrees max face angle error allowed

10 feet: 1 degree

15 feet: 0.64 degree

20 feet: 0.48 degree

25 feet:0.38 degree

30 feet:0.32 degree

For 10 feet straight putts, the putter face angle should be no more than 1 degree off from being perfected squared at the moment of impact.

For 20 feet straight putts, the putter face angle should be no more that half a degree off.