You really want to get a digital one that measures to +/- 0.1 PH. The one you pictured dosn't look accurate enough. If u cant get one locally i recommend you buy online. It may take a week to arrive but at least you have it for next time. And if u have spare cash use the $7.95 one as a rough guide untill a good one arrives.
Keep the water @ 6.5-6.7 PH