On a race face 68/73 external bb there are 3- 2.5 mm shims. I tried leaving one out and the crank has end play on my 68 mm shell. Is it possible or desireable to put all 3 under one side and none under the other side? On My 1x8 the chainline is less than desirable from middle front ring position to the 32 tooth large rear cog.

Well even though there is no replies I will reply my findings in case any other person has the same question in the future. With 3 stacked under on BB cup I was not comfortable with the amount of threads engaging my BB shell so I did not proceed with torquing it down. I think bad things may happen if anyone tried to do this and only has 3 threads holding one cup in the frame.

Race Face

Depends on BB shell + type of front derailleur.

I too am having the same problem. I used one shim on the drive side as indicated on the very sparse instruction sheet, but I have play. Backing off the preload ring eliminates the play, but it had to be backed off the non-drive side crank enough to disengage from the threads, which doesn't seem very stable, considering the stress it will be put under.
I have emailed RF and am waiting for a reply.
And yes, the cranks are fully seated, and yes the BB width on my bike was exactly 73mm, which called for using just one shim.
I'm thinking of using one more shim on the non-drive side which would allow the preload ring to remain threaded, which for some reason would make me feel better.

Shim the drive side to get the best line you can achieve in #4 or #5 cog (since you don't have a 'middle' with an 8-speed cluster) - - with as little shim as you can get a way with. Then add one to the non-drive side if you still need to take up some slack. Chainline to the big cog is just going to be what it's going to be - - set for middle.

BTW, check your BB bearings after a season and make sure they're clean and still spinning good. The stock RF X-type BB has that same awful plastic dust-cap/spindle interface that FSA externals have (so that they can cheap out and use standard 20mm ID bearings with the 19mm spindle. The dust-cap eventually at least partially fails, letting crap and corruption into the bearings. If they get notchy, replace them with a proper 19mm ID Enduro bearing set.

I will keep that in mind. Race Face just replied to an email I sent them and said the crankset was designed for a 10 speed set up and it would be hard to get an ideal chainline with an 8 speed. I will live with it. It works.