Mr. Ken,

Yes, you are correct on getiing the initial charge into the system to
over ride the low pressure switch to get the compressor to kick on.
But, how can one determine or get the correct opererating pressure's
without the engine running?
You can't.
And to get the right pressure's you must have the engine at fast idle,
with the AC set at max.
This is the proper way to get the right pressure while servicing any
automotive AC unit.
The hot water in a bucket trick will work well with R-12, any of the
aftermarket alternative's also.

The 134 I quit using a long time ago simply because it caused too many
failure's and problem's in older R-12 systems with lower cooling temps
at the vents.
Folks for some reason just don't like spending big money on a seasonal
luxury.

Happy B-Day to you Too.

RC