In an effort to help you enjoy your Londinium coffee in the way we envisaged we now use Volvic water for all our taste tests.
This isn't a plug for Volvic or that Volvic is better than any other water, it is simply a water with key attributes that are all 'in range' for espresso use (and there aren't many bottled waters that are) and Volvic is readily available as a result of an extensive distribution network.
We hope this standardisation removes one more variable from your espresso equation.
As a reminder; 8g of coffee per shot, grind fine, tamp light, and target an extraction time of around 22s. We could also add water temp measured at the group head of 92C.