I've just started using C8Pro and I'm having trouble using the grid in the spline modeler.  I need to draw a large model - 15ft long - but with accuracy down to the inch.  I tried adjusting the grid down to one inch increments, but when I zoom in to work on the details, the grid sets to a different measurement.  How do I zoom in, but retain the new grid measurements?

Any help will be greatly appreciated.  Thanks in advanced.