It's both more complicated than that and more simple than that.
shows typical C:N ratios of a variety of compostables. It shows grass clippings having a range of any where between 9 - 25 (C:N). It lists several different kinds of straw, I don't know which kind you have. So C:N could be anywhere from 48 - 150. So if grass clippings were 25 and straw 48 and you wanted a ratio of 30, you would use a little bit less straw than grass. If the grass clippings were 9 and the straw 150, you would use 3 times as much grass.
The trick is IT DOESN"T REALLY MATTER!
It will compost anyway. It isn't rocket science. So mix it more or less 50:50 ish and see what happens. If it starts to get slimy or smelly or matted, you have too much grass. So you add more straw, turn it a bit, and it will be fine. If it is just sitting there, not breaking down, it has too much straw and/or not enough water (remember it needs to be kept damp to work). Add some water and more grass, turn it a bit and it will be fine.
But I think you will get better composting and better compost (finished product) if you have more diversity of ingredients. Throw in your kitchen scraps and yard wastes!
I just put in what I have as it comes along, though I do always cover a layer of greens (kitchen scraps and weeds) with a layer of browns. But my pile tends to be more green in the growing season when I have tons of weeds etc, and fewer brown ingredients and more brown in winter when I have tons of fall leaves and fewer green ingredients. It works just fine either way!