Any vacuum gauge that will read to 20-25"Hg will do. Check the vacuum signal from the vacuum control valve (VCV) on the side of the IP to the transmission modulator. The like will likely have a green cylindrical damper.

I'm not sure what Brian means to adjust. I say this because a 93 300D, assuming that's what you have, doesn't have a transmission vacuum amplifier, a blue saucer in... well... who knows where it is in a RHD car. In my mind the VCV is something you set in a static condition once. You adjust modulator pressure to specification by turning the T-handle covered by the rubber cap on the modulator. Lift the T-handle past the castellated retainer and turn. To do this you should have the specification and a pressure gauge hooked up to the test port on the transmission below the modulator. It probably won't hurt to turn the T-handle CCW a castellated retainer notch or two to determine if that improves shifting. If your car has a transmission vacuum amplifier, then there is an adjustment point as Brian describes.

Sixto
87 300D