Hurtta Harness Sizing?

I'm wanting to buy Nola a Hurtta Y Harness, but the sizing is confusing the hell out of me! I don't even know if they make it in her size. She has an 11" neck and a 16 1/2" chest. Her Dachshund shape is hard to fit!
Also, from what I can tell the neck isn't adjustable?

Go by girth size first - they seem to run small, so size up if you think she's close to the edge of the size. The necks are not adjustable, the girths are, but the girth measurements seem more accurate on determining size vs trying to balance all the dimensions.

Going by that, she'd need a size 18, which is 16-18". Which makes the non-adjustable neck 13". That sucks. Maybe I'll look into the other one.