If you have a scale, then weight the arm-board and arm. Then and enough lead to flatten the sub-plinth and then take out the amount of lead to equal the arm board and arm. Then attach the arm-board and arm and all should be well.
Signed Man of Wisdom.
This is from memory. About 6 years ago I had a Sapphire. If I am totally wrong don't slam please. My ego is very fragile.