Plaster Pricer only includes the most common types of sheets of plaster. Eventually you will have to quote a job that includes plaster than is neither 10mm thick nor 6mm cement sheet.
To include less common plaster in your measure here are some options:
Include the different plaster sheets as a different measure
If you are entering a measure for a job at "82 Studley Park Rd" and you need to quote for some firecheck 13mm sheets, complete as much of the measure as you can (the 'common' sheets and all sundries) then save and close that measure. Make a new blank measure, and name it "82 Studley Park Rd 13mm Firecheck" - Make sure you include the address of the job - do NOT just enter 'Firecheck 13mm' as the job address so you can search for all measures by the job address.
Enter the sheets you need to supply as firecheck in the 10mm RE section, then save and close. Now there are two jobs for 82 Studley Park Rd in the job list - one calculates the area for firecheck, the other calculates all other areas.
If you have any other different types of plaster, create a whole new measure for them also.
This video show how this works: