You can also use the Erlang B formula for these reverse calculations.
You can use our free online Erlang B Calculator for such calculations.
We also sell a Windows version of this calculator for 80 US Dollars for a single user licence. This also includes an add-in for Excel that allows you to perform these calculations within a spreadsheet. Click here for more information. I have attached a screenshot of the Windows product to this message.
Please feel free to contact us if you require any further information.