Measuring/Transferring an Arc (Armhole) Measuerment

Hi all, I'm new I found an apron pattern I could use some help with if you don't mind. The pattern is a free pdf and is here:

http://www.michaelmillerfabrics.com/Blog/Classic_Apron.pdf

The pattern gives you the basic measurements which you cut from your own paper. You see the arc for the "armhole" there? How do I transfer that arc from the teeny picture to my full-sized paper pattern from the measurements given?

I really don't want to eyeball it if I don't have to. I know I can take it to a copy shop and ask them to enlarge it, but the rest of it is so easy I can do it myself for free, plus I would like to learn how if it's possible.

Is there a way to measure the "degree" of the arc so I can transfer that to the full pattern?

Oh, and yes, I'm anal

Thanks for any help,
Gina

You could use an apron you already have or borrow one from a friend?

Thanks for replying I don't have one to start with, no one I know has any either, lol. I know! I have never worn an apron, but now that I am cooking more, I'm getting tired of splatting ingredients on my clothes

Large dinner plate? I think you can do it freehand, just give it a try. 

Use string, a pencil and a pin. Tie the string around the pencil, put the pencil point down at the underarm mark and then pin the other end about two inches down from the corner of the paper. I think that will give you about the right arc and you'll just have to straighten the end of the upper line.

I ended up stumbling on another apron pattern that will work. But I did remember that I have an art projector that will enlarge that small image to actual size, and I could also use the grid method to transfer the arc. You make a grid over the small image, 1 square equaling an inch on the small pattern, then draw a grid on the actual size piece of paper with the same grid (same number of squares acdoss and down). Transfer lines from one square on the small image to the same one on the actual size pattern.

Once I stopped thinking about it as a sewing pattern, and more like an art drawing, those ideas came to me. Thought I'd share if anyone else was in the same boat in the future.

Thanks for your replies!