Given the prevailing winds - generally from west to east - the destruction to the west would only be a 100 miles or so. My guess, nothing more.
To the east, different story. Ash, measured in feet, a long ways from it. Like Nebraska.
Use Mt St Helens as an example. Complete blowdown and total destruction in what - 30 - 40 - 50 miles in the direction of the blast? I'd have to go looking again, but seems that I remember measurements of pyroclastic flows on the order of 100 miles or more. Again, I'd have to back that up with some research fact, for now, just some hazy memories.
While Mt St Helens was indeed an impressive event, it would not make a decent pimple on the ass of the order of magnitude of the two biggest events from YellowStone.
You say 100 miles. I'll have my toes closer to 1,000 - in any direction. With lots of cold beer. And hot dogs.
LVM