tan(angle) will get you the slope of your camera angle. Multiply that by the distance and zscale to get the z height at any given distance. Remember to also add the z height of the origin/camera as well.
Generally speaking though, Construct is still a 2d engine. A 2d engine with 3d elements features does not make it equivalent to a 3d engine. Making an fps is mostly an exercise in futility. You're only going to run into further road blocks down the line without 3d collisions, volumetric fog/lighting, sloped vertical surfaces ect. This is similar to the limitations of the Doom 2/Wolfenstein era, where you notably could not look up and down. Your effort would likely have better return if you spent it learning how to use an actual 3d engine like Godot or Unity instead if your dream is to make a first person shooter.
But if you do want to continue, I highly recommend looking in to some basic trigonometry and geometry. It will be invaluable for working in 3d space, and you're not going to be able to get very far without it.