fbherr

I already have the Canon EF+ Aperture >Fuji and the  lens has no helicoid , so you need that, the M42 to EF is a flat adapter.

I'd say use three. What the heck!...

... even then, the image circle would be greater than the one you need and the possible but unlikely lens tilt issue would be incredibly small and mostly only at very high apertures.
 
No problem then.
 
I fact I am about to embark in a project where I am using 4 adapters on a projection lens where I have modified an T2/M42 adapter, then I am using an helicoid M42/M42 then I am using a M42/Canon EF adapter and THEN I have an Canon EF + Aperture> Fuji FX adapter!
 
This is more of a fun thing and I am making use of things that I have already (and others that I am buying especially) to do all this. ( Check out my projection lenses adaptation thread in a few weeks)
 
Dust or particles is always an issue when using anything on a digital camera 

Alignment is the only issue I can think of really.

"If it works, don't fix it." Adapters are simply tubes with no optics. Using two instead of one should make no difference in the operation of the camera or image quality.

A quick format method that I have seen listed several places.  It works great for me.
 
I only format with the trash button.  I do not delete an image or group of images.
 
With camera turned on, hold trash button down for  3 sec. or so.  While continuing to hold down trash button, press the rear command dial  (the wheel at the upper right of the back).  The screen will ask you to choose slot 1 or slot 2.  Follow commands to delete all of the images on the card in that slot.   You must do the process twice, each time choosing the slot with the card you with to format.   Remember this process, by reformatting the card, deletes ALL images on that card.
 
Saves menu diving to clean and format cards.
 
 
With regard to uploading.  I use a card reader.  I download my images for the day and often continue to use the card the next few days, downloading only my new images for each day, usually to a separate folder in my main images folder.  I use 32 GB files which often last me two or three days when I am seriously shooting. \\
 
I shoot only RAW so my slot 2 is a backup.  When I have essentially filled the 32 GB card, i retire it and use another card to finish my trip, event or whatever.  The second slot contains a 256 GB for backup and this card is good to backup at least eight 32 GB cards.  At the end of the trip or event, when I get home I download my images via card reader from my 32 GB cards to my computer and to my backup external drive.  At that point I reformat, using the technique listed above for both all of the 32 GB cards used and the 256 GB back card.
 
I carry sixteen 32 GB cards and two 256 GB cards.  I have yet to run out of space even on a four week trip.
 
Maybe more than you all want to know, but perhaps this is a help or idea trigger for someone.
 
Jerry