There are a large number of variables in exposure of paper negatives on multigrade paper. Color of the light, reciprocity departure, even temperature can have a small effect -- and as you surmise, different brands of paper can be different, even different batches in the same brand. If Ultrafine hasn't chosen to publish an ISO equivalent, you're largely on your own to test and arrive at your own EI.
That said, a rule of thumb for modern enlarging papers is to try ISO 6 equivalent as a start. Keep your development consistent (i.e. if you choose Dektol 1+1 at 2 minutes at 20 C, develop all test negatives for that same 2 minutes at the same room temperature), and examine the negatives by scanning, photographing, or contact printing (so you can see them as a positive to check for shadow detail) unless you have considerable experience reading negatives.