Gibran2 has it right- broadly speaking, circulating blood volume correlates fairly well with body weight or mass (but if you want to be pedantic about things, there sre differences, albeit slight, relating to what percentage of an individuals weight comprises of muscle and fat).
For this reason, calculating the required dose is done sufficiently well by correlating to weight-this is perfect for IV dosing because the dose given is what eventually assaults the brian receptors.
Clearly this is less the case when it comes to vaporising as the method of ingestion as inevitably the dose vaporised will not be completely 'absorbed' if you will.
Referring to the IV route, bear in mind the route the active ingredient takes before it hits the brain and the significant dilutional effects which take place along the way:
Peripheral vein-vena cava-R side of the heart-pulmonary circulation-L side of the heart-aorta-carotid arteries-brain.
For this reason, IF it were possible to efficiently vaporise a measured dose of DMT and ensure full absorption (not really realistic, but bear with me) it would hit the user quicker than when IVed.This is because the route it takes to the brain is shorter than the IV route, namely:
lungs-pulmonary circulation-L side of the heart-aorta-carotid arteries-brain.
From experience with other compounds which can be IVed and smoked, I can confirm that this is the case.
I am paranoid of my brain. It thinks all the time, even when I'm asleep. My thoughts assail me. Murderous lechers they are. Thought is the assassin of thought. Like a man stabbing himself with one hand while the other hand tries to stop the blade. Like an explosion that destroys the detonator. I am paranoid of my brain. It makes me unsettled and ill at ease. Makes me chase my tail, freezes my eyes and shuts me down. Watches me. Eats my head. It destroys me.