So then the cap for dividend payments to shareholders is 20Mh/s per share. The excess will be used for electricity and buybacks.
There is no "cap" and seeing as though there are only 25,000 shares and more than 500,000MH/s currently on order, without a motion passing with a 66% 'yes' shareholder vote to issue more shares, the MH/s per share is guaranteed to exceed 20MH/s.
I'm still completely confused as to how this works, could you please rewrite the OP to show how much you payout? Do you payout 100% of your gear to 25,000 shares (with 18k of them held by the company)? Do you payout 20Mh/s per share and keep the rest for electricity (as the OP states)?