When the pump is disconnected from everything and vented, is the pointer at 0psi. If not, there are gauge pointer pullers available (an instrumentation tech will have one) that allow you to remove the pointer, reorient it to 0 and then press it back on.

After 0 is properly set, you will have to get a cal standard for whatever the max pressure of your gauge is and use it to monitor your reading at the max level. Adjusting this reading requires the disassembly of the gauge and making an adjustment to the full span adjustment link (This may not be present on guages costing less than hundreds of dollars) of your bordon tube inside the gauge. If there is no link, it may be more practical to use the pointer puller to put the gauge at 45 when your standard reads 45. This will result in a reasonably accurate reading at 45 psi but probably and inaccurate reading at 0. Since it is rare to fill a tire to 0psi this may proove to be an acceptable solution. Did I mention that all of this must be done in a container kept at standard temperature and external pressure (easier than doing all of the math to correct the readings.)

On the other hand, you might consider that 5% accuracy for a gauge in this price range is pretty good. 5% of 160psi being +/_ 8psi would point toward your gauge being ok.

And they said that bicycles weren't rocket science..............................