Hsn code under gst

Dear Experts and friends,

As per my knowledge, HSN code under GST is derived from HSN Code of Custom Tariff Act ( i.e. both are same) but under GST, maximum 4 digits are prescribed whereas in Custom 8 digits.

So If i am not getting HSN code for a particluar goods in GST, then first I see that goods in Custom HSN code and then take the GST HSN code under which that Custom HSN falls.

Is it correct ? Please advise.