Using the IV Calculator
First of all, pick the desired unit. Even if they are below 3 stars, you will be able to see the level 1 stats of a character from the "Unlock Potential" screen as long as they are above level 20.
Always make sure to equip the weapon with the highest available Mt. For example, you will want your clerics to have Assault equipped since it provides the most damage.
There are passives that can change the stats of a unit such as HP+5. To avoid any errors unequip any passive skills before using the calculator.
IV Calculator Stats Input Guide
If you are entering unit stats on the calculator from the "Unlock Potential" screen, there are a few things you need to make sure of or else the result will deviate greatly from the actual values.
In the case of this 4 star Robin (M). If you enter the stats as shown, the result will be incorrect.
This is because by default, Robin does not have the weapon with the highest Mt equipped, resulting in an incorrect Atk stat.
In order to prevent this error, you will need to know the highest available Mt weapon, you can do this by tapping "Skill List". Then find the weapon with the highest damage and add the difference between the Mt of that weapon and the base weapon to the initial stat. In this case Blárraven+ has 11 Mt while Blárraven only has 7, so you will need to add 4 to your Atk stat.
Lastly, some units have passives that grants stat buffs. Make sure to unequip these before entering your stats into the calculator.