Well just completed a pressure gauge on the in line to the RO/DI unit.
This was easy to do.
Got a rain bird pressure gauge that is made for the yard sprinkler system.
Then a hose male ended adapter or resizer from gauge to 1/2 inch female.
Then a resizer from 1/2 inch male to 1/4 quick connection which is the same size as the hose leading to the RO/DI unit.
Then got a quick connection "T" .
Cut the hose going into the RO/DI unit and then cut another piece 2 inches and put it on the now modified pressure gauge.
Put the "T" on the 1/4 inch hose going to RO unit and then attached the hose coming from the gauge.
Turned on the water and found that I have very good pressure of 80 psi's going to RO/DI unit.
If yours is below 40 psi's you should add a pump to the in line of the unit.
Pressure gauge on RO/DI unit 
