Thirty years ago, when this whole ultralightweight thing came around, we did not have good digital scales that were cheap. I used a 16-oz postal scale for the little stuff, then a 30-lb baby scale for a fully loaded pack. I would don that pack and step onto a bathroom scale as a check.
Now that we have good digital scales, I use a tiny 1000-gram digital scale (US Magnum), then enter in line items on a spreadsheet to get a total for a whole pack. I don that pack and step onto a digital bathroom scale as a check. The spreadsheet total weight is generally within a few percent of the actual total weight.