Breaking point formula

[David Love]

I'm looking at Roberts' formula for breaking point and he gives the maximum
as t(max) = .557d^1.667

He then gives an example of a string at whose tension is 447 lbs and whose
calculated T(max comes in at 556 lbs.  He then says that this falls within
the prescribed maximum of 60% of T(max).  Seems to me that 447 is 85% of
556.  Is the formula he gives incorrect?

David Love
davidlovepianos at comcast.net