What Level is recommended when gambling for this to get 30/20 block? Currently doing it with my Level 16 and I've had 2 amazing 2os rare bone shields come up but both with 15/10 block

How do I get more frequent 30/20 shields as I barely am seeing any.

I was gambling mine on 22. I think it depends if you want the 17fhr mod as well. 21-24 is ideal iirc.