My turn! My turn! London coffee crawl!!! 2.5 weeks!!!
Consistency is a difficult thing, I find... especially when there are 4 seasons in one day & the doors are open or even open & shut constantly. Also, it's hard to stop the line to hit the "perfect" shot with every single shot when there's a constant flow of drinks to be made and you know 10-15 more are about to come through in the next 5 minutes. If we 'see' a change in flow rate, volume, colour, etc, we'll repull the shot and if it's still 'wonky', we'll make "intuitive" changes and taste or if only a minor adjustment, taste at the next possible chance (usually, when a single shot is ordered). That said, flow rate/vol/colour, etc isn't ALWAYS the same... but we expect it to be similar 'looking' from the last adjustment (usually adjusted 3-8 x per day). In order to get it right, all the time, every time, we'd need to be slugging back shots every XX pulls... and it's just not possible
With straight shots or maccs though, I always try to get feedback. Straight shots tend to be regulars though and they know that if it's me behind the machine, they'll send me a signal (nod, face-scrunch, thumb up/dn) to let me know. I'd say maybe 1 out of 30-35ish shots per straight-shot-regular, I have to re-do for them