There is an area of collecting interest that depends upon strips of certain lengths. Collectors of plate number coils typically collect them in strips of 3 or 5, with the stamp with the plate number being the center stamp.
If it turns out that those are what you have, then there might be some premium for having longer strips (5 vs. 3, for instance).
Otherwise, for garden variety modern U.S., straight multiplication is probably in order. I agree with smauggie...a particular example would be useful, so that you aren't led astray.
C.
[edit: And welcome to the forum...

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