As maui mentioned, 2% is not necessarily a conservative value. For resulfurized grades under 2", the machining allowance should be ~ 3.4% for centerless turned or ground HR bars, and ~ 2.4% for machined bars. Over 2", the allowances should be 2.4% for either condition. Non-resulfurized grades under 2" should be 2.6% and 1.6%, respectively, and 1.6% for bars over 2". This information is from Ispat Inland Bar Products. MACSTEEL uses the generic 3% for resulfurized and 2% for non-resulfurized grades, but their round billet is not as susceptible to decarb as the corners of a square billet is. Ascometal in France is the only other round billet producer of which I know, but I don't have any comparison data for them.