If your goal is to make compost, i.e. arrive at a balanced C:N ratio, calculations could be made but they would have to be based on dry weight of the straw. For that you'd need the wet weight of a bale and its moisture content. I may be getting too scientific here.

Or, someone may come along who's done this before with a real simple answer.

Have you looked at what Pat Lanza has written about Lasagna Gardening?

Here is a link that might be useful: Lasagna Gardening 101

Thanks for responses. I had not thought in terms of dry weight. I googled a different search and located a great compost calculater--it was great because it made the math understandable. The site is www.greenwelcomewagon.com/tools/compost/details.php. My bales are just under 30 lbs so I'll be mixing in 2 lbs of blood meal for a ratio between 30 and 35--close enough for me.