This may well have nothing to do with your question David, but just in case...
Even a small amount of fruit acid is enough to prevent botulism,
so the acid levels in chutneys are purely up to personal taste.
For example, I like chutney to be quite high acid and not that sweet
I'd say commercial cider and malt vinegars have similar acidity, and I'd basically do a 1:1 swap.
I find homemade cider vinegar can be quite variable in strength,
but as you're cooking out most of the water, taste the chutney when it's the right thickness for you-
if it needs more acid, add vinegar and reduce the chutney for longer.
If you're concerned that longer cooking will wreck the chutney's texture,
just reduce some vinegar on it's own in a pan and add small amounts to the chutney until it tastes right.
