That blue tape (from 3M I think), has to be one of the best things for setting stuff out, no matter what you are doing.
A friend of mine is in an electric wheelchair, it failed near the local railway tracks last month and required 5 people to get her away from the train that was coming, in the next 30 minutes.
What had happened was, there was an alloy block that holds the motor to the body of the wheel-chair, it had worn out over time and a corner had snapped off of the block.
I had a block of stainless steel here I was going to use for something else, but it sat here for ages.
I called on my mate who has a milling machine, marked the thing out with the blue tape, back in the day we would have used "Engineers Blue", a liquid solution that used to stain steel blue during fabrication and you could "bruise" the steel to reveal lines and punch points.
Long story short, it's been a while since I've used a milling machine, and I wrecked the first bit I made, second one was drilled, tapped and fitted right into place.