The 146 performance spreadsheet available on Avsim does offer the ability to alter the parameters for flex temp take offs, and shows their affect on runway performance. I know its not perfect, but I'm in the process of checking it with my 146 and it seems to work reasonably well.
The spreadsheet is in the AVSIM P3DV5 Utilities section entitled:
BAe AVRO RJ / 146 Runway Performance Spreadsheet