On those rare occasions when I need to torque something higher than the limit of my biggest torque wrench, this is exactly how I've handled the job. You know how much you weigh, so just divide that into the required torque and now you know how long a handle you need.
But do note that what you care about is not the length of the handle or where to grip it relative to the end, but your horizontal distance from the nut. If you arrange the breaker bar or heavy ratchet horizontally, then the position at which you apply force to the handle and the horizontal distance are the same. But if the wrench is angled up at 30 degrees to the ground, then your applied torque will be low by 1 - sqr(3)/2 = 13%, by using handle length only. Use horizontal distance.
You probably remember your high school physics teacher talking about the "x-component" or "y-component" of a force vector, and this is it. Sorry if that causes PTSD for anyone.
Note: For those who don't remember 10th grade Trigonometry, the resulting torque will be W*L*cos(angle), where W is weight, L is length of handle to applied force, and angle is measured from horizontal. My sqr(3)/2 shortcut for 30 degrees stated above is just a shortcut on this, based on remembering the old 30/60/90 triangle side lengths.
