LTCS sounds like Low Temperature carbon Steel. This designation is too vague to be of any use and you need to find out exactly what material it is. Body material is your material for working out pressure rating, based on the class of the valve,, not the internal trim and seat material. Class rating is normally stamped on the valve plate or the valve body somewhere (150, 300, 600 etc). You normally find data like this in the valve list for a site. I think (hope) you've not phrased your question correctly as the class is predetermined by the flange and body rating, but the actual wax working pressure is governed by material, and design temperature.
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Also: There's usually a good reason why everyone does it that way