"Exact measurements" refers to precise and accurate calculations or values that allow for accurate and consistent results. It means getting the true and correct quantities or dimensions of something. Full definition
I definitely need to do some «stain / wash» sample boards to experiment with more exact measurements of paint and water and the resulting effects. (revolutionaryredesigns.com)
Note: I didn't use exact measurements for this recipe, so you can adjust to make as much or as little as you'd like. (thesoccermomblog.com)
It came out dense and the taste and it was good but had way to much moisture at the bottom, I used exact measurements with the unsweetened almond milk option. (mariamindbodyhealth.com)