This isn't elegant, and I KNOW i can do it with a loop.
but I figured it wouldn't be to cumbersome this way.

I wrote this function:
Function mm2in(Target As Range)
Dim mm, inch, denom, numer
If (Target.Value <> "") Then
inch = Target.Value / 25.4
denom = 64
numer = Int(inch * denom)
If (numer Mod 2 = 0) Then
denom = 32
numer = Int(inch * denom)
If (numer Mod 2 = 0) Then
denom = 16
numer = Int(inch * denom)
If (numer Mod 2 = 0) Then
denom = 8
numer = Int(inch * denom)
If (numer Mod 2 = 0) Then
denom = 4
numer = Int(inch * denom)
If (numer Mod 2 = 0) Then
denom = 2
numer = Int(inch * denom)
If (numer Mod 2 = 0) Then
denom = 1
numer = Int(inch * denom)
End If
End If
End If
End If
End If
End If
End If
mm2in = numer & "/" & denom
End Function

then put in cell B2:
=mm2in(A2)
in A2, i put the metric number  I want converted.

the INT function truncates the inch value.
You may wish to round it instead.