I always suggest people use zero bales per billion square miles. Straw is not needed. A water filled roller is much more important than hiding the seed from the sun with anything. I realize there are certain parts of the country where you would look like a freak if you seeded without using straw, but you really don't need it. Some places they burn their winter lawns every spring, so go figure?? The roller will push the seed and soil together so that the seed benefits from the ability of the soil to retain moisture. Be sure you water daily, 3x per day, for 10-15 minutes each time, for 2 full weeks or until you get what you think is 80% germination. Then you can start to back off the watering until the roots develop and you are watering no more than once a week.

Use the minimum you can get away with without looking freakish.

Thanks. Part of what I'm seeding is on a hill. Not real steep, but I'm hoping the straw will help keep the seed from washing away.

Any other opinions on how much straw to use?

A bale is going to cover somewhere in the neighborhood of 1000 sq ft. It doesn't have to be perfect and you want it thin enough so that you still see soil peaking through.