Hello!
The correct term for "above sea level" is elevation. The amount of elevation above sea level is also measured in feet.
Some ways to write "535 above sea level" would be: "Elevation: 535ft." or "Elevation of 535ft."
However, you can leave the phrase as "535ft above sea level," which is a more informal way of writing it.
I hope this helps you! Feel free to comment or PM me with any additional questions. Have a great day!