opening width/door width

Hi everyone,
Need some urgent advice:
I want to install a new door in single brick wall. Opening have been cut by a builder. The sides of the opening are not very straight. At the top the width is 870 but at the bottom it's 890. I can still trim the bricks a little if needed (hate doing that). I would like to make the door as wide as possible given the existing opening.

Say I want to install a standard door of 820mm wide.
Question is, do I need to trim where the opening is 870, to make it 890 all the way?
Or is it good now?

:?
Am I correct in calculating the required opening width like follows:
Standard maple jambs are 31mm thick.
Recess is 10mm each side.
Gap between door and the jamb should be about 3mm each side.

So on each side I need this space:
31mm - 10mm = 21mm for the jamb, plus 3mm for the gap = 24mm
So the opening needs to be 24x2=48mm wider than the door itself.
That means my 870 opening is almost perfect for an 820 door (820+48=868).
So it is not necessary to trim the bricks so that the opening is 890 all the way.
Instead, the 890mm bit will be packed when jambs are installed.

In other words, an opening should always be at least 48mm wider than the door to be installed.
Can anyone confirm that my thinking is correct?
Thanks heaps.


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