You may change this value by returning a different value in the [font=courier new]getMagnetizedLength[/font] method of the length unit(s) you want to use (like in LengthUnit.INCH#getMagnetizedLength). Replacing [font=courier new]precision[/font] by [font=courier new]0.6f[/font] in the computing of [font=courier new]magnetizedLength[/font] in getMagnetizedInchLength will do it in your case.
Keep in mind that length value are stored in centimeters so you'll have to return inches converted to centimeters. Note also that this rounding is used only when magnetism is active, is also applied to some other measured lengths like room sides and depends on the zoom factor in the plan in the current implementation.