We have helped a few clients set up companies recently.
The last matter required a special class of share for 15% of the total shares issued.
This entitled the holders to extra shares each time more shares were allotted to investors in return for capital, such that their original percentage of that class of shares remained the same, if they wished.
If you, or anyone you know, needs help with incorporating a company, especially with exotic share classes, see Michael Paterson & Associates.