Reddit user nalk1710 made a handy app that calculates optimal hyper berry use for legendary donuts in Legends: Z-A: This guide helps you figure out if it is possible to create each donut based on your inventory. It also suggests an economical (using fewer rare berries) and a luxurious (using rare berries, giving full power) recipe.
Update: not long after finding the legendary donut calculator, I found Pokéos’s Donut Maker which shows Flavor Scores, star ratings, Level Boosts, total calories, and potential Flavor Powers.