> is the computation similar to that of a 555 timer in astable op... 

It's linear

http://home.clear.net.nz/pages/joecolquitt/0f84rc.html

I've no idea if that graph is applicable to newer PICs, but
10k + 100pF @ 5V is about the frequency for a 16F88, so
maybe the RC circuitry is pretty much the same as always,
as is WDT

Some datasheets do have R and C vs Vdd tables. eg DS30487C
(16F88), Figures 19-11, 19-12, 19-13

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