Mick,
The commonly held belief is that rolling radius must be with 1% so you don't damage the quattro section of your transmission.
You currently have 1.33% difference with your set up which you may just get away with. Switching to 245-40/18 will increase the difference to 3.71% smaller on the front and you are heading for a whole world of hurt. If you want to run a staggered set up then ensure the tyre's rolling radius is within 1%.
I have a staggered set up on my projekt of 285/30-20 rear and 245-35/20 front. .07% difference, works fine. The arches are modified. Workable combinations and wheels for our cars is here.
There is a good calculator here and lots more information besides.
HTH
__________________
Conan (the Librarian)
Watch it if you dare
Currently,
2000 S8, Merlin Purple, Leather Pack 1, Winter Pack 1, SPAX adjustable suspension, AP Racing 6 pot front caliper, 385mm fully floating disks, EBC Yellow Stuff Pads, Black Diamond grooved rear disks and predetor pads, D2 Doctors Brake Block.
1997 4.2 A8SQ - the Projekt. 12 years and ongoing!
1997 4.2 A8SQ - Ming Blue. Restoration sat waiting; saved from the Scrapyard
2003 C5 Allroad 4.2Q - Cobolt Blue. Towing car. Laid up awaiting manual conversion.
2005 C5 Allroad 2.5TDi FE - Grey - Workhorse.
2015 Q3QS SLine - Daytona Grey. Lots of extras! Long Haired Admirals cutter.
Previously
2003 C5 A6 1.9TDI FWD - Loaner from D2 Doctor Lifex
1997 S8 - Ming Blue. Loaned out and written off. I loved that car. My first engine swap after cambelt failure.
1996 4.2 A8SQ - Ming Blue. 178k on a cambelt! Trader's 8. Delivered to A8Parts at Lifex.
2003 2.8 A8SQ - powder blueish. Fill in for a while. Sold on.
1998 1.8T A4 Avant FWD. Ming Blue (there's a theme here), PEX for the 97 S8.
And a list of non VAG going back 40 years before I saw the light. 
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