How to overlay the image scaling based on the user distance & focal length

I’m overlaying images on the cameraView and based on the user input of room width & height. The images are displaying proportionally to width & height ratio. but i need to overlay the images based on the distance. If 5feet means, then i need to display the images closer, if it’s 8ft means i need to overlay images littlebit far. I referred these links https://stackoverflow.com/questions/11913355/scaling-ar-pictures-based-on-the-distance-from-the-camera , https://stackoverflow.com/questions/13190932/how-to-find-out-focal-length-of-camera-in-ios-and-what-is-the-sensor-height?lq=1. But the result size is too small.

int imagesize_height = (152.4*199)/0.33;

 int imagesize_width = (152.4*350)/0.33;

distance in cm = 152.4 and ipadMini camera focal length is 3.3mm = 0.33cm, image width is 350 & height is 199