As promised, here is a video of me basically playing around with the pressure gauge and my spring.
I apologize for the ambiance, my machine is in a secret basement location until a kitchen renovation is finished. That could be quite a while.
To understand what's going on better, here's a pic of the face of my gauge:
To note are:
The very first tic is 1 bar. There is no half bar position for some reason. This threw me off at first because I thought the gauge was innacurate measuring my boiler pressure until I realized this idiosyncrasy.
Notice the locations of the 4 and 8 bar positions; they are bolded tics.
Here is some interesting stuff I discovered or verified through these exercises:
The spring maxes out at 8 bar, although it takes very little "help" to get it up to 9 bar or more if desired. (I forgot to film any "pushing" experiments, sorry)
When the lever returns to its vertical resting place, the pressure in the chamber is still rather high, around 7 bar or so. The pressure drops quicker toward the end of the shot. It isn't as linear of a descent as I had imagined. I imagine this would be even less so with real coffee since puch resistance is supposed to drop throughout the shot.
When lifting the lever, the pressure doesn't really start rising until at least the 9:30 position or higher with any air trapped in the chamber.
This means if you want to slowly ramp the lever pressure you should go especially slow toward the end of the lift.
That said, with a controlled lift of the lever it gets easy to "feel" when the lever is slowing down and is going to catch. I have gotten to learn through practice what "4 bar" feels like. This helps me hold the lever at the desired resistance for a higher preinfusion if desired.