How to add a high pass crossover at the amp inputs

Can anyone tell me the formula to determine the values of the parts needed to add a passive high pass filter directly at the Amplifier input. Also is this a good idea.

Thanks for your help.
Good idea for what? If it's to add a single-order to 'block' low frequencies from the subsequent speakers, it'll work.

A Search for 'filter formula' turned up this thread.