No, Zinn is all wrong about cranks. He suggests crank arms that are much too long. In my opinion most of us use cranks that are far too long, but he advocates much longer ones.
There are a few factors that matter, and leg length or femur length is really not among them. What's important is:
When riding a fixed gear, make sure your pedal can't hit the ground. If it does, you are leaving the ground and coming down sideways. When in doubt get shorter cranks.
Short cranks are advantageous at high cadences. Short cranks out more strain on the cardiovascular system and less on the legs and in particular less on the knees, except at very low cadence, when you might not have enough leverage to turn them over.
Long cranks are advantageous at low cadences. That's especially relevant on rough trails where you may have 'To power over rocks and things at walking speed. Long cracks put your knees through a wider range of motion which can be painful or even injurious, especially if your not used to it.
I don't recommend cranks over 170 mm for anyone except Zinn himself. I have 165s on most of my bikes because a vintage version of anything shorter than that is hard to find.
Once you get a hand on fg riding - strikes are easy to avoid and shorter track pedals will help too. I use FG offroad as well and as it is much harder to avoid strikes on ice, stones, roots, etc - prefer shorter cranks and slimmer pedals. But then again - I rarely get any strikes now compared to when I just started riding fixed everywhere.
For OP I agree - 165-170 is a safe bet but 175 would work too