If you already have a ledger and you buy a house or flat now then it is obvious how much does it cost. If you start a ledger now and you bought your flat 12 years ago then you might wonder what is its worth at the moment.
Your aim to use a value which is close to the real value but do not worry too much about it. Keep in mind that you only know the exact value if you actually sell your property. To get an estimate use a website where you can advertise you flat. For example in Hungary you can use the https://ingatlan.com.
Provide the city, the area of your flat (use a range, if your flat is 50m2, 43-57 may be a good range). Order the result according to price, select a flat in the middle of the list and use that price. If your flat has a great advantage (e.g. almost new) or a great disadvantage (e.g. the street is very noisy) then modify the price accordingly. Use another website, e.g., Facebook marketplace and check the ads there also.
You do not has to do this evaluation each month but once a year you might want to take into account the market change. In the meantime you can use a formula to calculate the value of the flat by taking into account the amortization, see the instruments tab.