Percentages are an acknowledgement that each of our systems are different. It's the better way to communicate a recipe. The same recipe can work for a brewer putting out 10G on a 3-vessel system and another brewer putting out 1.5G in a saucepan. This is the same as 'baker's percentages'.
Use software. Input your measured system parameters such as loss-to-grain, boil-off per hour, transfer losses, etc. These are unique to you and your process. The recipe writer had their own set of numbers. Your software should be able to spit out the proper amount of grain and water to arrive at your post-boil or fermenter volume at a certain OG. Practice and measurement over a few batches can really dial this in.