There are cases where we need to have the UV borders and edges placed precisely on a certain grid. an example of this is when preparing a UV layout intended for baked lightmaps in a game engine, that should preferably be aligned to a 64 by 64 or 128 by 128 grid.
To set the Unwrap UVW editor window grid to 64 by 64:
In the Unwrap Options window:
Set the Checker display to 64/20 which is 3.2.
Set the Grid Size to 1/64 which is 0.015625 (the numeric field displays the value rounded to 0.016)
Turn on the grid display in View > Show Grid.
In the Unwrap Snap Settings, make sure Grid Snap is checked.
* Hold and drag the snap button to open the Snap Settings.