Your savings rate is the percentage of your net income you do not spend: if you earn €1,800 and €270 is left unspent at the end of the month, your savings rate is 15%. It is more useful than watching your balance, because it does not depend on how much you earn but on how you live relative to what you earn.
There is no magic number. The 50/30/20 rule suggests 20% as a reference, but a 5% rate sustained for years beats a 30% rate that lasts two months. What matters is knowing your real number and nudging it up over time.
To calculate it you need two figures you already have if you track your movements: the month's income and expenses. And if a rough month drags it down, that is fine: you look at it, understand why, and keep going.