Trying out 'zone focusing' with NX100

So I'd like to try my hand at 'zone focusing'. While I could do it easier I'm sure with my D3100 and 50mm f/1.8G lens, I'd really like to use my NX for street shooting. It's less intimidating but most of all, lighter. As an ex-pat in Manila that means a lot to me!

Now I only have the kit lens (20-50mm f/3.5-5.6 ED) for my NX100 but I've managed to do the following steps easily:

Using this: http://www.dofmaster.com/dofjs.html

I calculated that at 24mm f/8 I have a 'zone focus' of between 6ft and 25ft, assuming my subject is at 10ft.

My only catch is that my lens doesn't have a focus 'ring' per-se that shows the focal distance. How do I set it for 10ft and ensure it's at that distance? Do I just find something 10ft away, get it into focus then leave it alone from then on?

[Edit: Minor spelling]