The use of a dedicated try gun will enable the fitter to arrive at a set of stock dimensions that suit and fit the Gun at the plate, on that day.
It will not replicate the handling and the Guns ability to shoot with a gun made to those dimensions, due a total difference in ergonomics.
A trygun is merely a tool to assist the fitter in arriving at a set of stock dimensions which MAY? be near ideal.
It is necessary to shoot the finished stocked gun at the plate or targets, and then to finalise the dimensions and then finish.
This very often is NOT done due to the high expense encountered.
Very often the gun purchaser is 'fobbed' of with technical jargon, otherwise known as 'bull***t'
If the customer already as a gun that they shoot well, it may provide a baseline stock setting upon which to work.
Remember the human form is highly adaptable and changable.
Many men (and ladies) look good in off the peg clothes, but they 'feel' better in custom gear.