Need unit specificationand length of line set.
Though someone will give you some sort of nonsense about this, the least amount of refrigerant you have in the system the better off you are in accordance with system design.
I know you don’t want to do HVAC evaluations, but if you measure the temperature of the line set before and after the liquid line where it goes from 1/4 inch to 3/8 of an inch you should not get a temperature drop.
This testing must be conducted under maximum design temperatures for your area.
If you don’t have maximum design temperature, then you are not required to perform the evaluation as this is not your job. If you want to cover your butt, you can always mention the fact in your report and advise the client that should the equipment capacity loss be observed, this is a possible solution to be considered.
1/4 inch liquid lines were commonplace and only units. Many of these lines pass through closed in walls and cannot be accessed for replacement.
Is the existing equipment a heat pump? If so, this can be more of an issue and if it is not.