Radio Shack SPL meters are damn accurate.
My boat came with a panel mounted digital DC volt meter that reads to .
1
I had a Fluke 187(some crackhead stole it!) that read to .001 and also
read AC on DC so I could see the ripple making it thru the rectifier
on AC Charge. It was fun to watch the voltage come up by the
thousandth.
The panel meter is .2-.3Volts low and is quite upsetting when it says
my 900AH golf cart bank is at 12.0 when It's in the high 12.3 with a
10 amp draw....

My problem with "dispose-a-tools" is that they need to be disposed of
at the most inconvenient times, when you are contorted into a tight
corner having spent a half an hour setting up a 2 minute job they
validate murphy and fail. argggh