Mak,
Please be more specific. Actually different vendors are using different formulae. based on the distribution of raw counters they have.
In some cases they include the SD blocking and in others they srart after SD assignment and SDR is used only. Probably this is what you asked, Vendors dont have the cause value of the SD request so they cannot tell you how many SD requested for Call_Setup were blocked so they dont include it. However some make the logical assumption that the percent of SD_Blocked is irrelated to which cause and all causes would be blocked once the SDs are congested so they just multiply (1-SD_Blocking) to the other factor so that it is also taken into account.