Im not 100% sure this suggestion will do anything, but the puck does expand when it first gets wet, which could slow the flow a bit.
If you pre-infuse then you may overcome this - sometimes the thermoblock machines will do it for you by pulsing the pump for the first 5 or so seconds. You can tell by the characteristic bzzz-bzzz-bzzz sort of noise at the start of the pour. Im not sure if the 4800 does this for you. If not, you can wet the puck by manually extracting for 3-5 seconds (so that no water comes out), then wait a few seconds, then start your 25s extraction.
Also, how old are your beans (from date roasted)? If theyre getting past it, you may find you cant grind them properly for a good extraction from the start.
And remember not to get too carried away by various routines - adjust your technique to how it tastes, not to how it may look :) Hope that helps - take some photos or report back with results!