Ok, design the A4 one first, and get it perfect and completely approved.
Then save a different copy of it for the 900x600 version, and move the items around as needed.
Shape is so important, and if they're asking you to design something that will be printed in different shapes ... well, that's definitely a hassle.
Do you know the exact dimensions of the corflute print?
As long as all the text and logos are vectors, and as long as you Placed the image (not copied and pasted it in) so it's a smart object, the whole design can be resized to any size you want.
https://www.damiensymonds.net/trainingprepress.html
Yes, sorry.
I'm glad you've re-done the calibration since this message, because that's what I would have told you to do.
I've never seen that one before, sorry.
Ehh ... I wouldn't have done it that way. I would have left all the pixels there, and controlled the size by lowering the Quality Level of the jpeg save.