I fail to see the fatal flaw, a lower grade (say grade 1) by definition a longer contrast range than a higher grade (4), so it takes more stops (time units) to go from white to black. If your f stop timer can handle it (mine cannot, it's "only" 1/4 stop steps) you would use 1/12 steps at Grade 5 and perhaps 1/3 steps at Grade 0, what is the problem?
Best,
Cor
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