Perhaps the social security system in Spain was designed before the easy availability of calculators. Instead of just taking a percentage of your employee’s salary, it divides the salary into a series of ranges, and each range pays a fixed amount.
For example, suppose you have a part time domestic employee making 268,80 a month. In this case you pay 49,29 a month and your employee 9,42 a month = 58.71.
If you give them a 1 euro raise, now you will pay 68.79 a month and the worker 13,14 a month = 81.93.
So the one euro raise will cost 23 euros.
I asked the person at social security about this, and they told me I should just use the by-law minimum wage per hour as the amount. (Implying to pay the rest in cash)