Brief Explanation of Midpoints please

Well, you know, I've tried to understand these, and I do. It's when (correct me if I'm wrong), for example, Pluto and Jupiter, the middle of them, that's the midpoint. And whatever house it is (let's say Saggitarius) then you find the planet of that. Which'd be Jupiter. But here's where I'm also confused. It can't be Jupiter. I'm not sure why, but that's what it says in my astrology book. I'm confused. Please help. I need to know:

- What do you when that (the example with Jupiter above) happens?

- How can i find a midpoint without using a computer or a calculator (maybe by estimating or something.)?

- Brief explanation of midpoints.

- Is it really nesscessary to calculate midpoints?
Again, thanks.

This link ( whit sub-links ) has the largest explination about midpoints:

http://www.cafeastrology.com/astrologytopics/midpoints.html
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http://www.cafeastrology.com/astrologytopics/meaningsofmidpoints.html
http://www.cafeastrology.com/astrologytopics/midpointpictures.html
http://www.cafeastrology.com/astrologytopics/midpointspictures2.html

You need to calculate it exactly, cause people can make mistakes and it's never the exact place of a midpoint if you use just your visual estimation.

I use this calculator, cause it's easy:

http://www.noendpress.com/pvachier/midpoints/index.php

PS: I think there was a previous post here from someone else, but a technical glitch made it disappear.