Concentration -- simple, just further reduce mouse sensitivity if hold_alt_fire. And I agree with you that this bonus does not apply to standing position. I think that's all.
If you want to make it more detailed ( but harder to code ), some other people also suggested if you move mouse too fast when in the "concentrated" mode, you lose the concentration and go back to normal mode.
If you want to make it more detailed ( but harder to code ), some other people also suggested if you move mouse too fast when in the "concentrated" mode, you lose the concentration and go back to normal mode.