There are 2 formulas, one is to find R2 first, the other uses R2 and R1.

According to fig 4-16 page 173 the graph shows 1uf & 10M ohm = about .2Hz. Also 10uf & 1M = .2Hz. Also 100uf & 100K = .2Hz

Doing the math LOW = .685(R2)C = .685 seconds no matter which combination of cap resistor you use.

Now I see bottom left corner of graph, R1+2R2 so what is this?

Charge time LOW is R2 only so mayby graph does not apply to R2 only applies to HIGH for R1+R2

Limits for R1+R2 is 3.3M but graph shows 10M

That means capacitor has to be 10uf

I am stuck. I don't remember how to transpose so I can work the formula backwards starting with the answer .1 to find R2