Thanks a lot for mentioning this website, it looks really useful.
I use http://www.webmetronome.com as a metronome. It can be customized as much as you like – number of beats (also ‘odd’ numbers likt 86 or so), accents, beats per cycle etc. The only disadvantage is that it sometimes ‘hiccups’ which can be disastrous when you’re playing to it.
Keeping the settings as simple as possible helps, as well as having only one browser window open and no additional online clients (FTP, IM) running.
(I’m not a harp teacher but perhaps this might be useful…)