As a machinist I constantly run across situations where I need to measure the center-to-center distance between two holes or pins so I can drill holes to make a mating part. The usual way is to measure the diameter of each hole to find their radii, measure the distance between either the inside or outside edges of the two holes, and then either add or subtract the radius of each hole to the distance between the inside or outside edges of the holes. A faster way when both holes or pins are the same diameter is to use a type of caliper that automatically takes the hole diameters into consideration as part of the measured distance between the two holes or pins. This is accomplished by taking the measurement from the inside edge of one hole or pin to the outside edge of the other hole or pin, which automatically gives the center-to-center distance. The caliper that does this has its fixed internal measurement blade reversed so it is facing the same direction as the moveable blade. Unfortunately, this kind of caliper is not made commercially even though there is a need for this design and it is very simple, but making one is not terribly difficult. The worst part is removing the fixed internal measuring blade because calipers are made from hardened steel and the blade can't be easily sawed off with a hacksaw. If you want to make a caliper, first try a good quality high speed steel hacksaw blade before resorting to an abrasive cut-off wheel to remove the fixed internal measuring blade. I suppose the blade could be ground off, but that would require keeping the caliper cool and the vibration shock of the grinding action dampened to a minimum to prevent damage to the delicate workings if it is a dial or digital type caliper.
When I made my caliper, I used a high quality Helios vernier-type that had been dropped by a machinst I was working with back then and the fixed blade had broken off. Using a bench grinder I ground the place where the blade had been to make the jagged break area flat and square, used a 3/16" (4.75mm is close enough) cobalt high speed steel drill to drill a rivet hole in the caliper body to fasten the new reversed blade on with, and machined a new blade from annealed 4140 steel to replace the broken one. High carbon plow steel or spring steel is plenty good enough, but a large grade-8 bolt is a good cheap source of 4140 steel for this project if it is annealed first by heating to first dull red heat and letting it cool down slowly before sawing out a flat piece large enough to make a new blade. I made the new blade slightly oversize to give me some "meat" to calibrate the measuring edge by honing it to the correct width and hardened it by heating to cherry red and quenching it in diesel fuel, sanded it with 320-grit sandpaper, and then tempered it by gently re-heating it until the first straw yellow color appeared. I riveted the new blade on tightly with a rivet made from a very soft steel #10 screw and then filed off the peened end to make it flat. Once the new blade was on I finish-sized the measuring edge with a fine diamond whet stone to match up the two blade edges.
To use the caliper, simply measure the distance from the inside to outside edges of the holes with the blades, and the read-out is the center-to-center distance between the two holes.
The top caliper in the photo is a standard caliper and the bottom one is the caliper I modified by reversing the direction of the fixed internal measuring blade by removing the original one and replacing it with a reversed one that I made.