Hello Stewart,
I was looking at Mazin Younis’s fine image of the nova (https://britastro.org/observations/observation.php?id=20240316_060436_8e8e3dc829f92b9d) and noticed it was a busy field, yet the VSP chart shows no bright stars that might contaminate the photometry aperture. The nearest is 113, but that’s still a fair distance. What does a visual sense check of the image suggest relative to the comps: 12 or 13.5?
The most recent obs at 13.2V on March 16.792, though that observer seems to report almost a mag brighter than others. What is the time of your observation compare? And just to confirm you deployed a V filter (the nova is much brighter in R).
JS