edited the applet to show actual reading = main scale + vernier scale - (zero error)
enjoy!
writing a journal article on this :)

contributed this to Wikipedia a while back and it is still there! http://en.wikipedia.org/wiki/Caliper
Zero error

The method to use a vernier scale or caliper with zero error is to use the formula 'actual reading = main scale + vernier scale - (zero error)'. Zero error may arise due to knocks that cause the calibration at the 0.00 mm when the jaws are perfectly closed or just touching each other.
when the jaws are closed and if the reading is 0.10mm, the zero error is referred to as +0.10mm.The method to use a vernier scale or caliper with zero error is to use the formula 'actual reading = main scale + vernier scale - (zero error)' thus the actual reading is 19.00 + 0.54 - (0.10) = 19.44 mm

Positive zero error refers to the fact that when the jaws of the vernier caliper are just closed, the reading is a positive reading away from the actual reading of 0.00mm. If the reading is 0.10mm, the zero error is referred to as +0.10mm.
when the jaws are closed and if the reading is -0.08mm, the zero error is referred to as -0.08mm..The method to use a vernier scale or caliper with zero error is to use the formula 'actual reading = main scale + vernier scale - (zero error)' thus the actual reading is 19.00 + 0.36 - (-0.08) = 19.44 mm

Negative zero error refers to the fact that when the jaws of the vernier caliper are just closed, the reading is a negative reading away from the actual reading of 0.00mm. If the reading is -0.08mm, the zero error is referred to as -0.08mm.