Sentence:  The length of the arrow indicates amount of slowing or acceleration at Completion over that at Onset , and the difference in months can be read <WSD>roughly</WSD> by referring the arrow to the age scale along the base of each figure , or more precisely by referring to the original data in the appropriate tables . 
Meaning of the word marked with <WSD> tags:  (of quantities) imprecise but fairly close to correct