For those before 1988:

A1=lowside;

A2=highside;

SE_a1=a*A1/100;

SE_a2=b*A2/100;

Ps = (ESTNUM - A1)/(A2 - A1);

SE = SE_a1 - Ps*(SE_a1 - SE_a2);

RSE = SE/ ESTNUM;

CI = t-value*SE;