Heres my undrestanding...
A3 sheet is two sheets of A4...so thats two times the size or 200% larger?
No, not really.
Its about 150% enlargement.
I will have to check the actual page measures to be sure.
Rotate a single A4 page and measure the sides.
Compare that to the A4 measure.
Looks like about 1 and 1/2 times the length on each side.
150%
If you rotated and printed your A4 pages onto A3 pages...
and assuming the printer enlargement was a straightforward enlargement based on page size...
with no "fit to page" or scaling adjustment...
then the final output is 150% of the original.
.............
okay, I just checked...
you are enlarging from 210mm x 297mm to 297mm x 420mm
which is 141% enlargment.
So...1:400 becomes about 1:284 scale?
or, am I totally lost?