Well yes, except you have to know the first value from which to start.
If you are airborne and you are given the QFE, you could add 14mb to determine the QNH.
If you were given the QNH and wanted to determine the QFE, you could subtract 14mb.
If you know neither the QFE, nor the QNH, you will be a bit stuck.
If you are on the ground at Sywell, you can set the altimeter to read zero ft. The subscale will show the QFE.
If you set the altimeter to read 424ft, the subscale will show the QNH.
The difference between the two should be 14mb.