I just recently went through a similar exercise. Planning a king size hunter's star pp'd quilt. It's only 2 colors. Drew it out in EQ5 and it came up with something like 15 yards/color! Granted, there is a fair amount of waste with paper piecing but that seemed quite excessive to me.
I went to Old Country Store in Lancaster, PA to purchase my fabric as my LQS had nothing that excited me and Old Country had one online that did. I'm so glad I went to see it in person as I really did not like it at that point! But I digress.
I found two fabrics that I liked and headed to the cutting table. Asked the very helpful young lady there for some guidance. She whipped out her quilter's reference guide (apologies - don't remember the exact name of the book and I even own it, lol). She looked at the backing fabric requirements for a king. Just under 10 yards total. Armed with that info, I bought 6 yds of the one and 7 of the other (it was the end of the bolt and why not).
But looking at the backing fabric requirements in that reference guide was an 'aha' moment for me. Thought that was a brilliant source as a guide for overall fabric requirements. Granted, tops need more fabric for seam allowances, etc. but I at least had a clue at that point. Now, frankly, I'll probably have to buy a bit more fabric than I did but I have selvedge info and that young lady also informed me that their online and brick & mortar quantities vary.
And I must say, that same young lady was an absolute gem. I mentioned to her I saw the initial fabric online but I'm a 'touchy, feely' kinda gal and wanted to see it in person. She gave me access to their website in store so I could find the exact fabric and then she ran her feet off trying to find that fabric for me! It took her quite a while to find it as it was tucked away being a 'fall' fabric. But she kept going until she found it! Also ran around pulling other fabrics for me that she thought I might like. Just a sweetheart! I've always had wonderful customer service from that shop but this young lady was exceptional.