I just did something similar on a laptop with the alpha version of Trisquel 8
but I think it's pretty much the same for 7.
The install program suggests a plan but select the option to 'do something
else' and click 'Continue'
Then it scans the hard drive and comes up with a graphic picture of it. On
mine, I had to delete all the partitions to create 'free space' (highlight a
partition and then click the - button)
then I could click on the free space to highlight it and then click on the +
button below. In the little window that appears you would
1. change the size from 8 to 4,
2. put / in the mount on text box and
3. select from the drop down box encrypted (I can't remember exactly -
something like 'physical volume for encryption,' it's the only option that
mentions encryption)
4. immediately new text boxes appear for you to type in your encryption
passphrase. (better not forget exactly what you put, of course I've never
done a thing like that, heh, heh, smirk smirk)
I also added a swap space the same way and it didn't like that so I had to
change that (there is a Change button too) to 'Do not use this partition' in
the drop down box.
I guess it would be best to leave the other 40 GB alone for now and let your
next project format it. But as you think best.
My 6 GB RAM probably won't need a swap, but I will have to learn how to
encrypt my swap space sometime or other I think because otherwise clear text
like data can end up in the swap on an otherwise encrypted system. I just
won't use it for now.
Also, another option in the window for creating a partition is primary or
logical. Primary is fine for what you have in mine. and until you click on
the install button at the bottom right, you aren't committed. You can delete
what you've done and start over. And 40 and 40 won't happen exactly but you
will get close. Depends on geometry of the hard drive or something.
Hope that helps.