Use the gross salary and gross pension (both are paid 14 times a year, so the percentage is the same monthly or yearly). Don't have a pension estimate? Use the pension estimator. The 70% target is a common reference for keeping your standard of living; adjust it to your situation.
The replacement rate is the pension divided by the salary. It shows how much of your working income the pension replaces; the rest is the drop you need to plan for.
The target pension is the salary times the target rate. The monthly shortfall is how much you would need to top up (for example with a PPR or other savings) to reach the target.
Educational estimate, not financial advice. It compares gross figures; the net replacement rate is usually higher, because the pension pays less income tax and no Social Security contribution.