Personally, I'd put the tack room across from the wash bay, not next to it, simply because if you have a leak, or even residual moisture seeping in, it can ruin your tack. (I realize that was just an example, 80acorns)
Also, I would consider dropping a stall or two and leaving an area open for storage, either for hay or various equipment, or whatever else comes up. There is ALWAYS a need for extra storage. Even if you have a loft, having space for a few bales on ground level is handy. But you likely would not need a 12x12 feed room for a barn that size. You could probably get away with an 8x12 with room to spare.
My barn is 50x36, my stalls are 10x12 and I do wish they were 12x12 but I wasn't funding the barn so I had to take what I could get. I have a 10x12 tack area and four stalls on on side and on the other, a 12x20 hay storage, a 10x12 feed and tool room (pitchforks, rakes, trimming equipment, etc) and then either 2 more stalls or more storage for shavings, etc, depending on my current number of horses. I wash mine at a pad outside, so we didn't have to worry about plumbing and drainage in the barn.
"Keep a leg on each side and your mind in the middle"