Paul, the best way to achieve accuracy is to take Frances's advice on using double transcriptions, where two people each transcribe the same page to Word or Excel or wherever, then compare and recheck any discepancies. It might seem like using twice the time, but it will pay off in the end.
I was interested to hear that you are using the original parish registers to transcribe from, rather than the films. If you can swap Bride for Maughold I'm happy to be the second person, and could compare your transcripts to Tom Corteen's ones from the parish register (same offer to the person who has Maughold, if no swap).

Sue