Math Challenge: Moulage Class with Kenneth D. King / Day One

To: 

all math geniuses, 

anyone with IQ over 140, 

nerds,

pattern-making experts

Subject: "Why are you still using inches in the US???" or "solve the problem!"

Yes, it is a Moulage Class review. But I won't tell you anything about it before we resolve this mathematic problem.

(Ok, I will talk about the class a little bit: Twenty-four measurements, fraction orgy, foggy brains and five hours later, and we walked out with a custom draft of the back bodice for Kenneth D. King's Moulage Class. The class is great - fitting and patternmaking bootcamp! but...)

Back to inches! Dividing fractions is (to certain extent) masochistic. You disagree? I mean it takes just a second to divide two-digit numbers in metric system! Now do the same in inches... Piece of cake? yeah, right... Now, try to use fractions for patternmaking / calculations. It can't be as accurate as the metric system.

Here is the very first measurement we took: my neck, which, by the way, measures 13" (or 33cm). To draft the moulage, we need to make the following calculation:

1. Neck: 1/6 (neck measurement) + 1/2  = back neck

let's write it out: 1/6 x 13" + 1/2 = 2 1/6 + 1/2 = 2 2/3 (here, try now and get it converted to eights or sixteenths for the ruler units). My calculation was somewhere around 2 5/8 (it is a millimeter more, but there is no way you can get this so precise for inches ruler)

NOW, LET'S DO THE SAME IN CENTIMETERS

neck measurement : 6 + 0.5 

to remind you, my neck measurement is 13" = 33 cm

33 : 6 + 0.5 =   5.5 + 0.5 = 6 cm

Now, look at your measuring band if it contains both, inches and centimeters. You will see that these two calculations differ from each other in slightly more than 1/4" (or appr. 7 mm). 

Or, am I doing something wrong??? Yes? Then, here is your challenge: prove me wrong! (and don't think of me as another arrogant European, please! I am neutral)


EDIT: Reader Jilly B found the missing 7 mm (read her comment after you found the mistake in my calculations). Thanks, Jilly B!

But my question still remains, am I the only one who thinks that fraction calculations are extremely cumbersome in comparison?