The Calculate button on the Plane Offsets dialog box (Insert | Feature | Constructed | Plane) allows you to calculate the offset values appearing in the Offset column when you change the nominal value.
For example, suppose you construct an offset plane between three circles (CIR1, CIR2, and CIR3). After you select the features and click the Offsets button, you have X, Y, and Z nominals of:
X = 6
Y = 2
Z = 0.95
If you change the X, Y, and Z nominals to:
X = 6
Y = 2
Z = 3.95
and click the Calculate button, the offsets for the three circles are updated to:
3.000000 CIR1
3.000000 CIR2
3.000000 CIR3
If you then click OK and construct the offset plane, you'll notice that the plane is constructed at 3.0 units higher in the Z axis.