Effectiveness is influenced by capacity of unit. If you are trying to mount a 270 cfm fan more than 30" above cooktop, then it may make noise but will do little to clear the air. If it is 1000 cfm, then it can be higher than 30". So the answer is "it depends". Most people don't mount them more than 36" above the cooktop unless it is a really high powered unit.

I mean that I want to put my stove in front of a window on the platform and the chimney or hood ventilation over it .what should be the height of a chimney over the stove .I mean I want to work standing in front of a window.

Sorry, it looks like in such a state of choas i hazard to give suggestions. But here i go any way :) The big window letting in lots of natural light...the sink goes there because checking dishes after cleaning them in natural light gives the best advice. If a stove will fit also then good...i prefer natural light when frying up bacon and eggs

A Rumford design is very shallow, has outward-flaring sides, a back that arches forward, a curve on the underside of the front lintel leading back to a broad slot-like flue with a flue chamber above it. The overall front opening is almost square, sometimes slightly taller than wide. Rumford fireplaces are very distinctive looking. You can get some basic information from this wikipedia article here: https://en.wikipedia.org/wiki/Rumford_fireplace and there is plenty of info online, even ready-made reproductions (more or less accurate). Rumford developed his improvements and published them in the late 1790s, just before fireplace heating was displaced by stove heating (a process already well underway when Rumford developed his improvements). Good luck with your whatever you decide to do.