If you are using a grid, I find that spells work best if you center them on a grid intersection (Pathfinder), not the center of a square (D&D 4e), no matter which style of diagonal measurement you go with. The spell descriptions in 5e just lend themselves to the former system a little more smoothly.
See paizo.com/pathfinderRPG/prd/magic.html under "Aiming a Spell" if you need further guidance.
In the end, you don't need an "official" method for this. (The "official" way to play is technically Theatre of the Mind, with grid play presented as an option.) Rules are easy to google for any style of measurement you prefer, and it doesn't significantly affect other aspects of the system, no matter what you pick. You just have to choose one that your group likes and be consistent.
See paizo.com/pathfinderRPG/prd/magic.html under "Aiming a Spell" if you need further guidance.
In the end, you don't need an "official" method for this. (The "official" way to play is technically Theatre of the Mind, with grid play presented as an option.) Rules are easy to google for any style of measurement you prefer, and it doesn't significantly affect other aspects of the system, no matter what you pick. You just have to choose one that your group likes and be consistent.