hope this helps everyone / anyone (if i've made a mistake someone correct me)
series II rb25det
the pipe below is where i spliced a small section out (no more than 5mm) i put the T piece in, connected the spliced pipe to either end of the T piece, attached piping from the last hole on the T piece which on my boost gauge (2nd pic) will connect to the new boost gauge sensor.
pic of the boost gauge (autogauge from just jap for those playing at home)
pic below is a mock setup of the boost gauge - im guessing most gauges these days come with the small circular filter (top right in the pic below) I cut the tube going from the black boost sensor to the T piece about half way - attached the filter (helps to smooth out the movement of the needle on the boost gauge / stops dirt/crap flowing back into the boost sensor)
lastly, pic of the location of the hole i used to run the tubing / wiring in under the drivers side kick panel then up through the steering wheel / under the dash etc ready to be wired up to the cig lighter / ignition etc.