how to avoid rounding rules when setting the grid size

[meta-ed]

I want a grid size of 1/8 inch. (Expressed as a decimal fraction, that is 0.125 inch.)

When Units = Inches, 0.125 rounds to 0.13.
When Units = Points (apparently 1/100 inch instead of the publishing standard of 1/72 inch), 12.5 rounds to 12.
When Units = Millimeters, 3.175 rounds to 3.2.
When Units = Meters, 0.003175 rounds to 0.0032.

How can I avoid these rounding rules and establish a grid size that corresponds to 1/8 inch?