Doing this for many years by now.
On each plug there's a ring. From the old plugs I take the rings off. With a Dremel I abrade the rings to different thicknesses. Say 1/3 to 2/3 of a millimeter. I use these rings to adjust the new plugs, leaving the new rings in place naturally.
Use these abraded rings over and over again. This works out fine and you don't need to use these rings on all the plugs. Only when indexing is way off.
I use the Champion QC12PEP plugs for my Evinrude 250 DI and they are very expensive.
Therefore I order them in the USA at Sparkplug.com, 24 pieces at the time. Only costs $168 and no shipping en no Tax.