The most practical way is to actually look at listings in your area - sites like OLX, Imóvel Web, and local real estate agencies give you a real idea of what's being charged for something similar to your apartment. Then you consider: exact location (near the metro, shopping?), condition of the property, whether it has a garage, if the condo fees are expensive. There's no magic formula, but most people use the average value in the region as a base and then adjust up or down depending on your place's features. If you want to be more conservative at first, set a price slightly below the average - you'll find a tenant faster and then you can raise it in the next leases.