That project uses an A1302 linear hall effect sensor. This is not the right sort of sensor. The web poster says it is his first project and it shows.
The problem with a linear sensor is that you have to read the value with the analogue input A/D converter. This takes quite a long time and is going to add errors to your measurement. You need to use a hall effect switch, this gives a logic level output that you can use with pulseIn().