One of the different factors to take into account when trying to determine the proper parylene thickness is the amount of clearance needed. If it is a printed circuit board that is an enclosure, there usually will not be too many clearance issues. However, in some cases, even an extra mil of coating can cause extra mechanical abrasion to the parylene which can result in damaged parylene.
Another factor to consider is the dielectric strength required. For applications that require higher dielectric strength, a thicker coat of parylene has higher dielectric properties than a thinner coat. Trying to balance the dielectric strength issue with the clearance issue is a tight rope to walk and will usually require some testing to determine the proper balance.
In the easiest of cases, an end item customer has specified the coating thickness to be applied and put this into writing on a drawing. After we review the drawing and compare the drawing with the assembly, we will be able to determine if any potential issues with the parylene thickness are present.