You are testing the hypothesis that plant growth is affected by the amount of water receives why isn't it a good idea to use a 8ounce measuring cup from your kitchen or a 12inch ruler

40 mm in um is a number that comes to beginner

Because plant grow very slowly so you will want a ruler with very small, precise and accurate measurements,

so you might want a rule that measures in milimeters (go metric system)

and the 8oz, that is a large measure, maybe use milimeters instead, that way you can more accurately measure the amount of water you are adding

by using those large measurements, you have a greater margin of error