How white does your white part of eye have to be? I heard you can determine your overall health by looking at the white portion.

To each his own. The whiteness of eyes can be subjective, similar to the whiteness of teeth. The importance lies more in the change from your baseline level of white. Over time, the whites of most people's eyes change, and they can also look more ore less white in different lighting or even when wearing different clothes. Some medical conditions can change the color, but it is not a great marker of overall health.
Yes. Yes. This is true, to some extent, but the degree of whiteness of the sclera (the outer eyewall) is only a clue to other systemic problems. For example, yellowing of the sclera may indicate jaundice, related to liver abnormalities. This will require further investigation by a gastroenterologist or hepatologist. Redness of the sclera may be associated with a wide array of eye problems.