Well...First you've got to be able to measure the distance from the breechface to the rear face of each slide lug...which requires either a single adjustable fixture, or three separate fixtures. I've used barrels and shim stock or feeler gauges for the task. Not as precise as an adjustable fixture...but workable.


Then, you measure the distance from each front barrel lug face to the unaltered hood face, and check against the slide measurements to determine
which lug or lugs would come into play if the hood was cut to just let the barrel engage with the slide.

Let's say that you have to remove .010 inch from the hood to get the barrel in...and that .010 inch coincides with the distance from the breechface to the first slide lug. Okay...You've got horizontal bearing on the first lug.

Measure from the hood face to the second barrel lug. Does the distance match the distance from the breechface to the rear face of the second slide lug? If it does...you've got lugs 1 and 2 in the game. If the distance is .001 inch short, you've got that much air between the lugs, and #1 is preventing #2 from touching. If it's .001 inch LONG...#2 is bearing, and it's keeping #1 from touching.

Check #3 in the same way.

If your fit is allowing #1 to touch..with 2 and 3 kissing .001 inch of air...shoot the gun. #1 should deform and set back .001 inch within 500 rounds and equalize the lugs. .002 inch should allow equalized lugs within 1500 rounds or so...and within another 500 rounds, they should be fully seated.

If 1 and 2 are bearing, and #3 is kissing .002 inch of air...you'll probably never get it to touch before you wear the rifling out of the barrel. If 1 and 2 are bearing, and #3 is .001 out...it'll probably take 15,000 rounds to equalize, and a little less if 1 and 3 are bearing, with #2 .001 inch out. More ammo will be required if you shoot mostly cast lead bullets.

Whatever amount of lug deformation and setback occur during the process of
fire...or pressure equalizing will be the amount of headspace that is added...so if you start with a static headspace dimension of .900 inch, by the time the lugs are fully equalized, that number will be about .902-.9025 inch, which is still a far cry from the .920 maximum.

Trying to pressure equalize lugs with more than .0015-.002 inch of airspace is pretty much a waste of time with today's hard slides and barrels. It will eventually occur with normal wear...but it'll take a long time. Filing the barrel lug faces to get the dimensions within that magic .001 inch is the way to go, but it does require patience and skill with the file to prevent cutting the lugs
at an angle from side to side. Due to the barrel's tilt into the slide, the lugs will probably start to seat at the lower area first, and "come in" toward the top with use, and become fully seated and equalized.

Because the barrel tilts at the rear, vertical depth of engagement will be greater on #1...a little less on #2...and the least on #3. Depending on how far the barrel tilts when tightly engaged vertically...the actual figures will vary a little from gun to gun...which is why it's not a major issue if 1 and 2 are engaged horizontally, and leave #2 in the breeze. #3 offers the least engagement strength. Nice to have it lend a hand...but not all that critical.

Even if you only have #1 horizontally engaged...as long as it has full vertical depth, or is at least 95% vertically engaged...the gun will last a long, long time as long as you stick to mostly standard-pressure ammo, reserving the hot-rod stuff for testing and carry duty. Using reduced pressure levels for the majority of your shooting...especially if you use it with cast bullets...you can go broke handloading enough ammo to cause enough deformation and increased headspace to be a concern. 230-grain cast bullets loaded to 750-770 fps will probably take upwards of 75,000 rounds to do it.