I know there are many definitions of a unit of enzyme but the definition i was given in class is "the amount of enzyme needed to digest 1ug of DNA in 1 hour at optimal temperature in a 50 uL reaction volume."

We were given a question on an exam where we were to calculate the amount of enzyme needed for a 20 uL digest. I took into account the reduction of volume and had points taken off because my professor said that the new volume did not have to be taken into account. My question is when does the final volume of the digest become a factor in how many units of enzyme you add? Using my professor's answer, I could perform that digestion in a bath tub as long as I keep the amount of enzyme and DNA the same which cannot be true. Any help would be greatly appreciated!