thanks very much for the detailed description - I think i'm getting it .

So initial flatness is from the flatness of the 'true tool' you lapped the work to (I still think 1/4" thick would be a challenge given finger pressure alone will affect how the work contacts the lapping plate) and you have three so they can be checked and touched up when required? .....and the checking of the work is done via an optical instrument, an optical flat?

sorry for the million Q's, optical work is out of my realm and but i'm intersted in how its done