This is a collection of the calculators I have created during my career as a DB.
Apple XP Cost Calculator - Calculator for determining the amount of Magic Crystals (aka Apples) required for re-birthing, leveling, as well as the amount of time required to farm said apples.
Damage Calculator - Used to calculate damage as the title implies (Not accurate for support based skills).
Devil Development Cost Calculators - Calculates the cost in Macca to fully develop a demon; includes Rebirths, Demonic Equipment, and Dsouls. There are two of these; one of them allows you to choose the rates per path, and the other is less advanced.