I know i'd know the difference between a 3.5mm and 5mm stroke machine. I have an adjustable direct drive machine and at it's longest stroke (5mm) i find it very difficult to tattoo with smoothly but when i adjust it to around 3.5mm stroke i find it tattoos much better. I can see how someone wouldn't be able to tell the difference between a 3.2mm and say 3.8mm stroke with just the naked eye and feel. But a 1.5mm difference is a lot.