Kevin's .022" sounds right to me. I've successfully used up to 1mm, and .022"=.55mm

Do you have a waste gate? If so, for the boost gauge, tap the waste gate line from the turbo outlet with a tee for the gauge, or for that matter, any manifold tap upstream of the throttle - woops, diesel and no throttle - any tap, including the brake booster line before the check valve. I would avoid any metal drilling unless you are SURE you can get ALL the chips out.

Not responsible for advice not taken...