Default difference require in characteristic

Dear Exprerts,

   I am new in VC , require help to solve the requirement.

charac.

V_THICK_N_TOL   Thickness Negative Tolerance

V_THICK_P_TOL   Thickness Positive Tolerance

   My client want V_THICK_P_TOL value default will be diferrence from V_THICK_N_TOL.

V_THICK_N_TOL= V_THICK_P_TOL+.7 mm

like , V_THICK_N_TOL = 1 , V_THICK_P_TOL=1.7

  What should i write in procedure

Thanks in advance,

Learner