Do radiators need to be installed exactly level?

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I'm a newbie here, and also a newbie at radiator installation, so please forgive my probably rather foolish question...

I'm replacing rusted steel panel radiators in my 1900s house, where many of the walls are still the original lath-and-plaster.
After consultation with she-who-must-be-obeyed, we are installing the modern version of column radiators.
In some rooms, the radiator-sizing calculator has dictated 4-column radiators 600 W x 830 H as there isn't the space for a longer rad.
These are very heavy, and there's no way the 4 brackets provided would carry that kind of weight on the old lath-and-plaster.
And as they're 4-col and the original skirting mouldings are wide, the rads need to be further from the wall than with the standard brackets.
So, I also bought the legs that are designed to carry these radiators, plus some radiator stays to provide stability / prevent movement.

Now I have a different problem, in that the floor is (of course!) not horizontal, so the radiator is out of level by 20mm in 800mm.
Is this a problem from a plumbing perspective?
Or do I need to pack 12mm under one set of legs in order to get it truly horizontal?

I'll of course move the bleed nipple across to the high end, that goes without saying.
Also, the radiator drawing indicates there's an internal "blocker" at the bottom between the last 2 columns at the outlet end.
This is clearly to force the HW up to the top of the radiator before it drops down the last column, to ensure it gets hot all over.
Makes sense, but it also makes me think there can be no plumbing issue with it being 20mm out of level, only the visual issue?
And as all the furniture will be sitting on the same sloping floor, I don't think that will be an issue either.
IMHO it might look more odd if the radiator was the only horizontal thing in the room!!

Of course, I'll need to consult the boss again, but I'd like the benefit of your collective wisdom before I do that....

Thanks in anticipation of some good-natured merriment at my expense.....
 
Also, the radiator drawing indicates there's an internal "blocker" at the bottom between the last 2 columns at the outlet end.
This is clearly to force the HW up to the top of the radiator before it drops down the last column, to ensure it gets hot all over.

Are you sure that’s not the inlet side? (I’m unsure.)
 

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