The subtract 1 or 2 inches is because Klein's frames are sort of in 2 categories: 1990 to 1993 and 1994-1996. When they changed frame designs the seat tube length changed so as I remember, if it's a 1990-1993 frame and you're trying to "convert", subtract 2 inches but if it's a 1994 - 1996 then subtract 1 inch.
They also didn't have a 21" size until late 1993 so their frame sizes were like this:
___'90-'93______'94-'96
XS __18"_________18"
S ___19"_________19"
M ___20"_________ 20"
Lg___none ________21"
XL___22"_________22"