[mrlucky0]

Say you have a pure isomer of glucose that rotates plane polarized light X degrees, does this mean it's enantiomer will light -X degrees? What if you flip just one of the -OH (form a diastereomer), in this case, is it not possible to determine the degree of rotation?

[g-bones]

yes, to the best of my knowledge, the enantiomer will give you the same degree of rotation with the opposite sign.  I am also fairly confident that the diastereomer is unpredictable from the rotation of a particular enantiomer.