Got to agree with Midden about not using masking tape. I have done this professionally for a few years and have an NVQ 2 in sealant application(for what that's worth). I would shave a point with a Stanley knife on a short length of wood the width of the desired bead, cut the mastic tube nozzle at a 45 degree angle at the same width. Making sure that the surface to be sealed is squeaky clean and dry. Where you've already had a go I would cut all mastic out with a blade or chisel leaving the mastic in the large gap. Then lay down the bead of mastic from the gun, stopping every time you took another pull on the trigger to keep the flow consistent. Doesn't matter if the bead looks a bit lumpy at this stage. Then, take a handheld sprayer filled with water and a bit of fairy liquid and spray it all over the mastic bead. then, spray the end of the wood you've prepared, hold it at about 30 degrees to the mastic one end and shave it off running the length of the bead, flicking the residue on the wood into a container with a splash of water and fairy to prevent it sticking. You don't have to do this in one go, you can stop and flick off the mastic when it starts to pile up on your tool you've made. This really will give you a professional finish and if you foc it up, wait for the mastic to go off, cut it out and try again.
BTW...Top marks for cleaning the tap. :lol
BTW...Top marks for cleaning the tap. :lol