Found this cool spreadsheet that you can edit to get a good idea of the minimum DPS required from each damage dealer based on your raid size: https://www.reddit.com/r/wow/comments/5v063y/minimum_dps_spreadsheet_external_link/
I would suggest changing the Tank and Healer weights on the "vars" page to better match what damage your healers and tanks in your raid group do. For example with Group 3 ours are probably 0.3 for tanks and 0 for healers.
- Heroic Krosus
- 2 tanks/3 healers/8 damage dealers
- Tanks 0.3
- Healers 0
- 399k DPS minimum per damage dealer
- 145m damage total per damage dealer