I like the comparison “4m short of each”, Peggy! 
Because in the large arena, it’s 2 m short of each.
So, if I keep in mind that it’s just double the distance I’m used to between where I “touch” and the next markers (KDF/HGM) then that should help me. And the distance I’m looking at is just 6 feet versus 12 feet… that’s something I’m comfortable with, I can grasp that… I can just “see” the distance of two ponies end to end between us and the next markers… I just can’t “see” 10 meters from the end of the arena in a concrete way that I can grasp from the back of a horse while trotting and moving his shoulder over and half halting and asking for more bend etc.
You could go out in an open field and ride a 20 m circle.
…I could, if I could do it accurately on my own two feet! 
Neither the horse nor I can do it on our own, we need lots of help… I think another few years worth might do it 
You do that by riding endless numbers of correct circles
LOL…I think you missed the part about “my “eye for a distance” is pretty blind.”) And, I’m pretty happy when schooling I can get at least one good circle in each direction… that’s a big accomplishment for a rider with no “eye for a distance” and a horse with stiff shoulders, etc!
I’m already planning to reconfigure the temporary markers to make the long arena into a short for my next 3 practice rides before the show. But, that’s not enough to fully compensate for our shortcomings. This is the only show in the area with a short arena.
In the meantime, I just wanted some strong visual to make it accessible to me, because “half way” and “2/3rds” just weren’t translating. And I’m sure we’re not the only horse/rider combo out there who can’t ride lots of perfect circles repeatedly, so I thought others could share the “crutches” they’ve used on the way to being able to do perfect circles. 
(Just in case I made it sound like we’re complete incompetents, our last show we got a comment “nice bend and shape of circle”, so we’re definitely improving. Then the next ride I wasn’t watching my landmarks and we got commented for what that did to our circle shape! (In that arena, my landmark is to watch the rafters: two line up great for the imaginary lines (KDF/HGM). I think the second time I had my head down rather than looking forward, so, bye bye round circle.)