Lots of ideas. I'll add my two cents.
If the part is accessible, I will measure distances between two holes by inserting tight fitting pins. Then I measure the diameter of one of the pons and zero the caliper. Next, measure outside to outside distance. The caliper reading is the center to center distance. If holes are different fiameter, it becomes more tricky. You have to measure both pin diameters, add them up, and divide by two for the average diameter, Then, with the caliper zeroed out, measure the outside to outside distance and subtract the average diameter for the center to center distance. Careful measurements will give you the center to center distance within a thou or two. For threaded holes, I use screws instead of pins. Screws have a relatively loose fit so I will use short socket head cap screws, tightening the down to the surface of the part and measure off the diameter of the cap. Shoulder bolts also work well. If neither is available, I will run a nut on each of the screws and tighten to to part. This helps to center the screw in the threaded hole. Finally, if a nut isn't available, measure outside to outside distance and inside to inside distance, taking up any loose play, and average the measurements to get the center to center distance.
A similar process can be used by measuring the holes with the inside jaws of the calipers. It is potentially less accurate than using the pins because care must be taken not to have the jaws set too deep in the holes as that will prevent you from measuring a true diameter or distance. When measuring the holes, you have to add the average hole diameter to the inside to inside distance.
Usually, distances are nice even numbers. Designers like to design like that. Check the dimensions in both inch and metric if you don't know the origin of the part. But don't assume that to be true without checking. I was fitting a new back plate to an Asian chuck and measured hole to hole distances to determine the pattern for the three bolt and found that the holes missed the perfect 120º bolt hole pattern by a half a mm. Measure all distances to get an accurate layout. I like to lay out the measured dimensions in a drawing for a sanity check.
When working with parts that can't easily be blueprinted, One method that give absolute alignment is to drill and tap 6the first hole and mate the parts by fastening them together with that hole. I align the parts for a good fit and using the second hole as a guide, spot the hole with a tight fitting drill, followed by the tap drill and tapping. Insert a fastener in that hole and move on to the remaining holes. It is more time consuming but it onsures a perfect fit even using hand tools. This process works well for dowel pins if you use a reamer for the final fit. If I am making both parts, I will make the first hole in the "pattern" piece for a tight fit for the fastener and drill the remaining through holes for the tap drill size. Then I drill all the tap drill holes, followed by opening up the hole to a clearance for the tap , and tap the holes. This process uses the clearance holes as tap guide and insures perfect alignment. Finally, I open the through holes to a clearance hole size for the fastener.
When blueprinting complex parts, I have spindle mounted microscope that can resolve to a tenth which I can use with the DRO to accurately blueprint a part. I have successfully used that to make gaskets for carburetors. It makes for a great optical comparitor. I determine coordinate points for all features, collect6ing as many as nec3essary, and locate the set of points in a drawing then I connect all the dots to make the pattern.
I had thought about building a spindle mounted microscope for the Tormach for some time. The idea was to cannibalize and old phone or adapt a webcam camera but the optics was rather daunting some the project was shelved. About a month ago, I saw an ad for a USB 50 - 500X microscope on eBay for...
www.hobby-machinist.com
