Looking for explanation of choices on "Prepare disk space" step in 9.10 install

[David M. Karr]

I have Windows installed on a box.  I want to install 9.10 on this box 
so I can choose which one to boot at restart.  I'd like to partition the 
single disk so that more than half of the space is available for Ubuntu.

I'm a relatively experienced Unix user, and I've been running 8.10 on 
another box for a while. That box is also running Windows, which I can 
choose at startup.  I don't remember what the installation choices 
looked like for that old box, but the instructions I get when installing 
9.10 are definitely somewhat different.

I could make some guesses in the install, but when making partitioning 
decisions, I really don't want to guess.  I've looked through the 
existing docs that I can find, and I really can't find anything that 
really makes it completely clear exactly what I need to do. The 
instructions on the Ubuntu site aren't clear enough for what I would 
think is an extremely critical point of the installation.

So, on the "Prepare disk space" step, I have the choices of "Install 
them side by side, choosing between them each startup" and "Specify 
partitions manually (advanced)". This page also seems to imply that the 
default partitioning will divide the disk in half.  I see that the 
little graphical area that shows how the disk will be divided has a 
movable divider, but I don't see anything that clearly states that 
whatever value I set it at will be used for the "Install them side by 
side" option. When I tried moving forward with the "Specify partitions 
manually (advanced") option, it replaced the simple partition map with 
all orange.  The next page seems to give me the ability to repartition, 
but it seems relatively complicated, and I don't see clear indications 
on that page for exactly what I need to do.

My drive is about 320gig.  My best guess, if I had to proceed without 
any confirmation, is that I should be able to move the slider so that 
about 90g is used for Windows, and the rest for Ubuntu, and then the 
"install them side by side" choice will automatically repartition the 
drive that way (preserving the current contents of the drive, where I 
had Windows installed) and continue the install.  I can't proceed from 
this step unless I'm sure this is what it will do.

Can anyone confirm this for me?