Large RPM and Vacuum drop when shifting from park to gear

Just suck on the line;if the can is good, the rpm will go up.

Cans are rated from about 5 to11, in distributor degrees, and the arms are stamped to reflect this. You double the number for crank degrees.
Inside the nipple is a preload adjuster that you can delay the onset of timing. But I noticed that if I crank mine to the end, it also does not deliver it's rated amount. I can't say if they all do that.

If you map your mechanical timing as degrees to rpm
and your Vcan as degrees to vacuum,
Then you can determine at any rpm and load setting, what advance your engine is getting.
IDK how useful this is information can be to you..... jus saying.
I used it to help determine my cruise timing. You might hardly believe how much cruise-timing my engine liked.