Hydraulic brake pull (or push?) ratios

I have no idea about hydraulic brakes!

Do they use a standard "push" ratio, the equivalent of a cable brake pull ratio? For example, all canti brakes work on the same pull ratio, as do all V's. And all road brakes too (apart from the very newest ones, I think). So with cables, it's easy to mix and match.

Is it possible to mix and match hydraulic levers and calipers? or do different makes use different ratios (volume of liquid moved / mm of pad movement)?

I started wondering the other day about the possibility of combining the new shimano di2/hydraulic road levers with the sram hydraulic rim calipers... I'd also be interested to know if older MTB magura levers could be combined with a more modern disc caliper, or are they limited to the original rim brake version?

thanks!

A lot will depend on whether they are sealed or open systems. Different piston diameters will give varying amounts of travel and force.

I think it's something best approached by playing and seeing what works - fun ideas though to run mixed road and MTB groupsets for touring, which is badly served by component suppliers.