I'm not sure how you're describing how you are getting it from the vent itself. Are you digging a tunnel that overlaps a wider section, allowing a safe channel down with pumps attached?
If you really are concerned that you won't have enough to flood however much area, build/dig a large cistern out and fill that up first. I would recommend a cistern anyway, as you can position it to your liking and build in a structured manner, as opposed to the organic form and supply of the magma vent.
Magma on its own generally flows rather slowly, but when it is pumped it is as fast and expansive as water, provided it has a constant and full supply.
I'll try to describe it better.
The bottom Z-level "magma flow" tiles (the ones that produce the magma) are very narrow, say, 5 tiles wide at the edge I want to pull from.
In order to use a pump, I'll need to create open space above the tiles I want to pump from.
The next z-level up from these magma flow tiles the pipe is much wider, and extends further south by about a tile than the level below it, so I cannot create "open space" above the magma flow tiles, as there is more magma in the way on the level above.
All I'm concerned about is whether digging a hole in the wall next to the magma flow tiles on the bottom Z-level and using it to fill a small area that extends past the magma on the floor above (so i can channel down into it) will slow my magma pumping down.
It's hard to describe, I guess.
A cross section of the pipe would look something like this
7 - magma, | - un-mined area/wall, # - "magma flow" tiles.
z1 - |||||77777777777||||||
z0 - ||||||||#####||||||||
So you see, I can't channel directly to the # tiles, as there is magma on a "shelf" above.
So what I have planned is something like this (_ is a dug out area, w/floor, P is where I want my pump stack to start.)
z1 - ||||_P |7777777777|||||||
z0 - ||||||__#####||||||||
The magma should flow out of the magma flow tiles, creating 7/7 magma for the pump to pull up.
I'm just wondering if this will be slow.
Sorry for the rough drawing.
You do not really need to flood whole map - the fire will take care of every living thing.
Hm ... I wonder - if you came in adventure mode and pull the lever - will you burn (not flood) the whole world?
Of course I need to flood the map, or at least the lower levels
I want this to be as excessive as possible.
If I get it to work I will definately try it in adventure mode and try to live long enough to see what happens.