How about like this:
Take the Max pressure 400 psi/ 600cc = 0.667 psi/ 1cc
If the tip of the glow plug took up a volume of 2cc; removing that volume would be a loss of 1.334 psi from the Max pressure. Max pressure without the glow plug tip would be; 398.666.

He can take the actual pressure reading he gets devide it by 600cc and use that amount of psi per 1cc ounce he knows the volume of the glow plug tip and subtract it from the total reading that he gets.

__________________
84 300D, 82 Volvo 244Gl Diesel