ok, it looks like i still have work to do !
i can NOW locate china radio on 45w, but at the sacrifice of 30w 34.5w etc....i can find it, but only by "fudging" my USALS by almost .7 of a mark on longitude....
looks like my ARC is not as sweet as i thought - very odd though, i would have thought being able to go from 53east to 53west, my arc would have been bang on....
I had to slightly adjust (AKA "fudge") the arc on the Gibby by about 0.3-0.4 degrees to get a really solid lock on China Radio on 45W, as well as almost all the other sats across the arc - but those are minute amounts which, as I said earlier, would be almost impossible to eliminate by the adjusting the setting of the motor on the pole.
Don't forget that, as the dish sweeps further and further E or W from your Southern-most sat, the sats are further and further away and thus, whilst the
angular difference between them doesn't alter as far as the motor is concerned, the actual "physical distance" on "your horizon"
does increase a bit - and therefore so the dish needs to be pointed more precisely at the orbital position (and the signal gets weaker as well in most cases - especially when on the footprint fringes).
TBH, I don't think you'd get "much better" after a lot of long and tedious, and often counter-productive, work as the "law of diminishing returns" will come into play.