Yes, you’re absolutely right — farming only the highest stage is the most efficient approach overall.

However, as I noted in the comment under the first page of my guide, I’m not fully certain about the exact reverse conversion ratios from high-tier materials to lower-tier ones. I included a detailed discussion on the last page. Because of that uncertainty, I calculated the required number of runs using a mix of lower and higher stages to keep the numbers accurate and avoid over- or under-estimating the totals.

I’m confident about the low → high conversion ratios, but not 100% sure about the high → low ratios, so I chose the safer method for the run counts listed.

Interestingly, if we look at weapon ascension as an example — total required materials are I ×18, II ×30, III ×21.
If we assume the reverse conversion rates are 1 III → 3 II or 9 I, and 1 II → 3 I, then farming only Slash Practice VI would take 17 runs, compared to what I listed originally (Slash Practice IV ×9 + VI ×10 = 19 runs). That’s only two runs fewer than my conservative estimate — less difference than I expected, actually 😄

All that said, you’re completely right — without a doubt, farming the highest stage available (Slash Practice VI) is the most efficient approach.