This is correct
What you have discibed is the correct way to do the job.
You may find the locking ring is tight,when i did mine i cut some ring's of cardboard and covered the lens incase i slipped.
You will have to try a few time's before you get it right,but start at 10 yds and see what your gap's look like.
The gaps should slightly increase from about 12mm from 55-50 and get progressively bigger as you get towards 10 yds.
If you get big gap then small gap it will need to be re-done.
About as much use at field target , as a life guard at the Olympic swimming pool.