So I bought a pair of 1Ks and think I may need to size down another 1/2. I can slide my foot forward and fit a finger behind my heel with casual socks. This is my first pair of boots so I don't really know how to gauge what sort of room should be left for foot expansion. Thoughts are appreciated!
My advice is to put in a boot insole cut to size, Red Wing does some, but there are heaps out there, and see if it works before returning them.
Honestly, this is my sizing advice in general: size down 0.5 and put in an insole.
The 1ks come with a hard insole. There's basically no padding or give. If you intend on wearing them much, you'll find that an insole makes the experience a whole lot better.
There is a reason that good hiking shoes/boots, and workboots used by actual blue collar workers have an inbuilt cushy insole these days.
Anyway, this is just my experience (2.5 years of wear on mine). I am sure that others will beg to differ