There is a familiar formula for the attenuation of a voltage divider formed by two resistors: A = R2/(R1+R2).
Not so familiar are the formulas for determining the value of R1 or R2 when a specific amount of attenuation is desired.
Too often I find myself having to apply some algebra to invert the formula.
Occasionally I get it right the first time without mixing up R1 and R2.
To eliminate this minor irritation, here is a calculator that determines the unknown resistor value.