Within FHWA Circular No. 7, see Eqs. 5-38 and 5-39 on Page 91.  The yield strength, fy, should come from your soil nail supplier.  Many soil nails are Grade 70, but the designer needs to know or specify what is to be used.

what do you mean with allowable nail design load?.

each failure mode has its own allowable design load which may be demanded differently thru the wall.

the key is designing all the different failure modes consistently (let say, the bar should not have 200 tons capacity when the shotcrete just resists 20 tons)