Hey Mr. Dean.
Most people that I've been talking to have been going with just a single pack of Alpha for their Blazers.
A handful went with two packs and either covered their roof entirely to prevent flex (again, while it does work really well at preventing flex, it's not what the product was intended for) or used just a little out of the second pack and shared the rest with someone else or a buddy. Really doesn't require a whole lot.
So with that said, I'd go with a single pack and go with 25% coverage.
Thank you very much Jon.