Mouton,
I think we will have to agree to disagree on this one.
There are a LOAD of variables you have to take into account when saying whether another part of the system is effected by a change in volume and it includes the internal diameter of the pipes used, the rate of change in pressure (flow rates in other words), the sensitivity of the pressure detecting devices attached and even down to the design of the T-pieces used for connecting in (which play a very important part of it, the T-piece is a special design for my gauge). Personally I think it perfectly OK to install the boost gauge as we have done. In my case I followed the manufacturers fitting instructions, I know it's not always adviseable to follow what a manufacturer suggests but they wouldn't specifically tell you to Tee the sender on the Fuel Pressure regulator vacuum line if it was not safe to do so.