On page 20 of bq20z95( http://www.ti.com/lit/ds/symlink/bq20z95.pdf ), same measurement has two dimension marking. For example 4.50/.30 & some others. What does that means

 

That's 4.50/4.30
minimum and maximum dimensions in millimeters.

That's an amazing chip! 44 pins of, "Knows every milliamp hour into and out of 2 to 4 lithium cells with less than 1% error at every point in their life, during every charge and discharge, at any time or temperature..." How did we survive all these years just charging our batteries when they got weak?

 

then what value ideally I should take. Should I take center point like if 4.50/4.30 then I should take 4.4mm

 

Use the standard footprint for that package in your PCB software as your IC use a standard IC package. But anyway this will guide you.
http://www.cypress.com/?docID=31451
If you are unsure just print the footprint out on paper in size 1:1 then place the chip ontop and see how it fits. But remember EDS protection then doing so

 

You shouldn't be designing a circuit board or anything for that matter until you understand simple tolerances and how they can/will effect the final end product.