I've recently (last 2 months) started doing farmer's walks on my non-lifting days along with some sprinting both for conditioning and to build strength in a way that I think translates well to real life. Finding a weight my first time was basically by trial-and-error. I wanted a weight that I could carry about 50 yards before my grip gave out and that I could repeat about 6-8 times.
Don't ask where those numbers came from, they just kind of intuitively felt right for me based on the fact that I wanted to concurrently build strength and give my heart a workout. From there, I progressed first by adding a few more passes and then once I got to 10 carries I increased the weight by 10 lbs per hand. I've gone from 100 lb db in each hand to 150 lb in each hand over that time period.
I try not to look too much at what "standards" are for my weight/height/age in different lifts instead, I start at something that's challenging and slowly build from there. Obviously if you are doing them more for conditioning you'll want a lighter load that you carry for longer and vice-versa for strength. I'd just suggest starting light and adjusting as you go. Sorry, I know this isn't scientific at all and probably not what you're looking for but it's worked for me. Great idea though on adding them to your routine, they've worked wonders for me in a relatively short time frame!
P.S. I've always loved your avatar, Bunny is one of the best characters in any war movie out there imo