[Elecraft] K2 frequency counter question

[Jim KO5V]

I have what may be a stupid or naive question: has anyone used the built-in frequency counter in a K2 to measure the frequency of a circuit external to the radio? 

I found what may be a reference to this, done by a French ham, but his link no longer works. If this is possible, any info (especially potential gotchas) would be greatly appreciated. I have built a couple of nixie-tube clocks as Christmas gifts, and I need to adjust their internal frequency references. The output is a 400khz, TTL level square wave. I know I probably need more precision that the K2's counter may have, but right now, I just want to get as close as possible.

I hope I'm not completely crazy here...

Thanks. 73,

Jim KO5V