Less flow back in the reservoir means more pressure in the line to the brew head meaning higher brew pressure right?
So my 4 ounces flow back meant a bit less that 9.0 bar which was good which means I'm being counterproductive here - way to go Alex!

I'm going to aim for 4 ounces which corresponds to approx 8.75bar. I'm trying to reduce the bitterness in my shots (and in my shorts!).

Here's some suggestions:

Go to Cafe Myriade, say hello to Scott, & buy some beans.

Read the review of the Andreja Premium on this site (HB) and get your brew temps down. Set your flow back to the reservoir as I previously indicated - take several measurements as a double check on yourself.

Ok so the flow is back at approximately 3.8 ounces. Funny you say get your brew temps down, it always seems to me the brew temps are consistently low, which would explain the bitterness. I always thought espresso was supposed to burn the lips when you drink it immediately after the shot. My shots are more on the comfortable very-warm-close to hot scale.

EDIT: February 21
So what you're suggesting is that the bitterness came from temps that were too high. I have tried flushing the group longer yesterday evening to get the temps down and have changed the boiler pressure to 1.1 bar.
I think I'm sensing less bitterness but only had two shots left of coffee so I couldnt experiment further.

I ran out of black cat intelligensia so I will get some good coffee this week and report back - again!

Thanks for the help!