I don't see why you would need to drill a hole, that pre dates the jiggle pin that eliminated the need for it.

The gauges on these aren't that accurate, mine sites between the 'M' to 'R' under normal conditions.

There are but problems with the senders in more recent times where they are not the correct type, which is what results in the gauge "reading" higher.

You need a capillary gauge, if you want accurate info.

Martin