Yes the level will depend on the input line level. 100K was my first guess, but in reality even moderate-heavy clipping is okay when counting frequency, it doesn't change the frequency of the signal any as far as the N810 is concerned. Be sure to post pictures and your findings when you're done!

Instead of mouser, you can also cut up one of these:

http://www.dealextreme.com/details.dx/sku.7396 (~3 weeks to ship)

http://cgi.ebay.com/3-ft-Mini-AV-3-5...QQcmdZViewItem

http://cgi.ebay.com/3-Mini-AV-3-5mm-...QQcmdZViewItem