What is the simple rule behind these three relations below?

$$F$$ $$\rightarrow$$ $$C$$ :: $$R$$ $$\rightarrow$$ $$I$$ $$\rightarrow$$ $$\sqrt{F}$$ :: $$IA$$ $$\rightarrow$$ $$D$$

translate to number $$A\rightarrow 1,B\rightarrow 2$$ etc. turn upside down $$6\mapsto 9,18\mapsto 81$$ etc. and take the square root.
$$6(\mapsto 9)\rightarrow 3$$
$$18(\mapsto 81)\rightarrow 9(\mapsto 6)\rightarrow \sqrt 6$$
$$91(\mapsto 16)\rightarrow 4$$