How much do I need to contribute (monthly)?

There is no mandatory contribution. For a pension you work back from the monthly income you want later: set the amount per month, and the pension calculator shows the matching contribution given your age, retirement age and expected return.

Goal-based investing works the same way: you enter your target amount and end date, and we calculate back to a monthly contribution. The more you contribute, the sooner you get there. For a pension, keep an eye on your jaarruimte, and make sure you keep a buffer available.