I have an Ensembl Transcript, ENST00000029410 with a mutation CT at a 1-based position of 808. Mapping this transcript to a protein is very simple, as the transcript's position is its position in its coding chunks, so the mutation's position on the protein is simply ceil(808/3)=270.
This transcript also maps to 3 Refseq transcripts (according to Ensembl's Biomart): XM_006714816, XM_006714815, and XM_005265805. I assumed a RefSeq transcript (XM or NM) represents the entire Ensembl transcript that maps to it, so I expected the position 808 on ENST00000029410 to also map to position 808 on each of the three Refseq transcripts. However, instead they mapped to three different positions: 833, 1044 and 1319, respectively. Where are these positions coming from? And how can they be used to find the mutation position on the resulting protein? Clearly dividing these positions by 3 does not result in a position of 270 on the resulting protein.