o-ring groove, the chart and the lathe disagree

Groove for a 1 inch OD o-ring. Chart says a width. My parting blade is wider than the chart and narrower than my patience. I can take it in two passes and risk a step, or grind a blade to the number. The number is in millimetres on the chart and inches in my head. Sorry.

Grind the blade to the width. Two passes leave a rib the o-ring will find. Convert once, write the inch number on the chart, stop converting in the cut.

Ground the blade. Groove is one width. O-ring sits in it and doesn't look offended. The inch number is written on the chart in pen, like a local.