ditto, dead simple reaction, but dead easy to f*** up as well.
Be consistent, always use the same measuring cylinder, ballances, glassware etc to keep the errors consistent. Don't measure different chemicals in the same glassware, always keep things as seperate as possible.
If you'r carefull and do everything 100% right it's quite easy to get readings accurate to about +/- 0.1s, especialy if you use something other than your eyesight, (measuring absorbtion of a particular wavelength of light or temperature work quite well with accurate equipment).
Ohh and measure/controll everything, no point mixing up a batch of chemicals with tap water, then expecting the same results the next day when theyr at room temp. Stick them all in a waterbath at 40deg.