How do I determine safe distance from a choke to prevent accidental coupling?
It depends.  And there are no hard and fast rules.  The biggest factor is the design of the choke/inductor.  There are some inductors that can be placed right up against some steel with no ill effects.  Other inductors, in the same design, might have major problems.  The type of core has a large effect (air cores are probably the worst, torroid is the best-ish), but there is enough variability that it is hard to give good rules of thumb that always work.

In the end, you need to try it and see what happens.  Take the prototype, turn it on, and wave a piece of steel around and measure how it is affecting your device.  You will learn more in 10 minutes of doing that then you every will reading answers like mine!

That being said, here is my experience:  I design a lot of DC/DC converters in the 1 to 50 watt range.  I use the typical off the shelf power inductors with powdered iron cores that are suitable for this sort of thing.  Metal has to get within about 0.1 to 0.2 inches away from the top surface before there is a noticeable effect.  But, as I said before, your mileage (kilometerage?) will certainly vary and don't take my word for it!