When brewing coffee, approximates will not lend to a good or consistent final brew. Using coffee-to-water ratios for any brewing method will allow you to have a target final weight and a constant extraction for every coffee.
Typically, Cartel's suggested ratio is a target weight (water) within a specific amount of time. Reference our Hario V60 brew guide. We typically stick with a 1:15 or 1:17 brew ratio –– 1 gram of coffee for every X grams of water. It is not uncommon, however, to see ratios varying from 1:14 to 1:18 depending on the roast, profile, and philosophy of the shop. Ratios with lower numbers typically correlate to a higher concentration in the final brewed coffee. Large batch brewers tend to benefit from lower ratios and controlled manual drip devices benefit from higher ratios.
If you do not possess a gram scale, a good rule of thumb is 1 tablespoon equals approximately 7 grams.
In addition to pre-weighing your coffee, you should weigh your water in real time when possible. For manual brews, weigh the water while pouring from a kettle. For automatic drip, weigh the water before you pour it into the brewing reservoir.