I'm also curious to know what others do. I keep track of time per game/passage doing exactly what you're suggesting. Marking the beginning of game and end of the game. My watch has a bezel, so my times are a little easier to tell.
For the PreProBono Fellows, we're currently using this watch.
@YZ, whatever happens, save 2 form the 60's just in case. If by 1.5 weeks out and you're not breaking 170, you should push it back to October. Worst case, you do just as well as ...
I think you can find it for less than $50. I was going to buy this one which is $25 but has mixed reviews: http://www.amazon.com/LSATTimer-Analog-Watch-LSAT-Prep/dp/B006A9SCU0/ref=sr_1_1?ie=UTF8&qid=1370480448&sr=8-1&keywords=lsat+watch
An *unpublished* study found it helped college students prepping for the LSAT: http://www.nytimes.com/2012/04/22/magazine/can-you-make-yourself-smarter.html?pagewanted=all&_r=0
The guy in this video claims to have gotten a perfect 180 and did so by redoing about 20 different practice tests repeatedly (there were only about that many PT's available when he was studying). http://www.youtube.com/watch?v=_sOsE0OlR00