[Orgmode] agenda prefix - fixed/max width

[Eraldo Helal]

I have set org-agenda-prefix-format to: (org-agenda-prefix-format " %-8:c ")
making the agenda look like so:

 home:    [ ] brainstorm redesign home
 evening school: [ ] chose DE spec.
 evening school: [ ] ask Lea re: spec topics
 evening school: [ ] brainstorm Learners Group
 eraldo:  [ ] brainstorm 4 perspectives

If I change it to " %8:c " I get that:

    home: [ ] brainstorm redesign home
 evening school: [ ] chose DE spec.
 evening school: [ ] ask Lea re: spec topics
 evening school: [ ] brainstorm Learners Group
  eraldo: [ ] brainstorm 4 perspectives

but what I want is this:  ;; limiting prefix to 7 characters

 home:    [ ] brainstorm redesign home
 evening: [ ] chose DE spec.
 evening: [ ] ask Lea re: spec topics
 evening: [ ] brainstorm Learners Group
 eraldo:  [ ] brainstorm 4 perspectives

In other words: having a fixed prefix length resulting in showing only
the set amount of characters.
Why? Because it lines up the check boxes "[ ]" which is handy and
looks nice for printing.
I have some long category names / file names ... making the prefix
verrry long no matter what setting I use at the moment. :(

Any ideas how to fix that?
Would be awesome if someone could help me get there. =]

Greetings from Austria,
Eraldo