During extraction, glomalin concentration is higher in first extract and decrease further. Therefore, a single BSA standard may not be suitable for whole glomalin estimation. I would recommend to use multiple standards. By the way, 10 to 300 mg/l BSA range is sufficient for the complete estimation. I would also suggest to use Bradford instead of Lowry method because later sometimes over estimate glomalin

Make a 200 ml stock solution of 10 ug BSA/200ul PBS (10 mg/200 ml), aliquot into 1-ml microcentrifuge tubes, and freeze, until needed. (Larger amounts of the stock solution may be prepared. Part of this larger stock solution or a portion of the 200 ml solution may be placed in bottles and frozen. The frozen stock may then be thawed and with 1-ml aliquots made at a later time to reduce the amount of 1-ml tubes stored at any one time.) Good Luck...CAN