Fuzzy logic in Prolog

Question

I'm trying to obtain a % to have covid, based on number of sympthoms and number of contacts (strict contacts or not). I read about using a CLP module in SWI_prolog, this is what I did, but I'm not sure I'm using the fuzzy logic, probably just random math.

:- use_module(library(clpfd)).
sintomi_fuzzy(X,Y):-
( X in 0..0 -> Y is 0 ;
  X in 1..2 -> Y is 0.45 ;
  X >= 3 -> Y is 0.6 ).

strict_contacts(X,Y):-
( X in 0..0 -> Y is 0 ;
  X in 1..2 -> Y is 0.5 ;
  X >= 3 -> Y is 0.8 ).

no_strict_contacts(X,Y):-
( X in 0..0 -> Y is 0 ;
  X in 1..2 -> Y is 0.3 ;
  X >= 3 -> Y is 0.4 ).

find_prob(Patient,P):-
       # S is  the Y from sympthoms
       # C is the Y from strict contacts
       # C1 is the Y from contacts
       sintomi_fuzzy(2,S),
       strict_contacts(1,C),
       no_strict_conctacts(0,C1),
      (C1 > 0 -> P is (S + C + C1) / 3 ; 
       P is (S + C) / 2 ).

How can I implement fuzzy for obtaining P?