Thanks for all the input, at least I now have a confirmation that the RS16 can't do it and now I can stop trying to work out what I'm doing wrong
The main port reading is definitely flawed and the balance port is more accurate regardless of the quality of the main charging leads, so it's the easiest way to ensure consistency. In my case just charging 1S with no balance port gives me a result of 4.15V but I can compensate with the TVC setting. My balance port is also not totally accurate, it's 0.02V per cell out when charging 2S, but again I can compensate with TVC. TBH, these minor variances probably don't notice in performance but maybe it's my OCD side that would like to see it right.
Anyone know if the Orion or Corally chargers can use the balance port on 1S?
TIA