Take the camera out and find a scene with a fence or a long wall that recedes into the distance. Point the camera down the length of the fence. Turn the camera knob to .8. Measure .8 m from the camera down the wall or fence. Check the ground glass to see if the camera is in fact focused at a point .8 m in front of the camera. If yes, then you have verified the accuracy of that setting. If no, then note the true plane of focus and write that down. From then on, you will know that if you set the knob to .8m it will be in sharp focus at distance X. Repeat for all of the other settings on your knob. When you have gone through the settings once, you will know where your camera is actually focusing whenever you set the knob to a specific setting.
You will need to test the infinity setting on the camera. Pick an object in the far distance and focus on it. That should get you close to the setting. There is a formula based on the lens you are using to determine how far "infinity" is, but for now, focusing on a distant object will get you close enough. The camera may not be all the way back (minimum bellows draw). There may be a fudge factor built in.