What are the 3 variables that according to Fischer Black

What are the 3 variables that according to Fischer Black

Question
Read the article Forex risk and the wealth manager.

What are the 3 variables that according to Fischer Black any investor should consider to calculate the optimal hedge ratio for FOREX risk?

Why you should hedge only a fraction of your portofolio for FOREX risk and never 100% of you r portofolio?

What is Siegel's paradox?