Incorrect or improper drive air pressure can have a dramatic effect on handpiece performance and turbine longevity so it's important to get an accurate reading.
Many practitioners like to boost performance by turning up the drive air pressure, but high pressure (generally in excess of 40-45 psi, but specifics depend on the make and model of handpiece) can also dramatically shorten your turbine life. Many feel the trade off is worth it for the increased performance, but be aware there is a trade off.
In order to really get an accurate measure of drive air pressure, you can't just look at the gauge on your unit. The pressure shown there comes directly from the block and can often be off by as much as 10 psi. This is why I recommend using a test gauge to measure your drive air pressure. Compare the reading on the gauge to the reading on the unit gauge to determine the difference. Fortunately, this difference is generally consistent, so once you know how much your unit gauge is off by, you will know where to set your drive air in the future.
To use it, the gauge should be installed between the end of the hose and your handpiece.
A test gauge can be purchased through most any dealer.