Sensitivity analysis with tax

Should we include the before-tax PV of Cash flows or after-tax PV of Cash flows to calculate the sensitivity of sales?

I know that Contribution should be after-tax (tax should be deducted) but what about PV of other Cash Flows?

I had a question in the exam where after-tax sales were being given with the tax rate of 30% & we were asked to calculate sensitivity analysis of Sales. So I could not understand that why do we take the after-tax sales for calculation because tax is charged on operating profits or contribution but not on sales!!!

Please tell me what is the way that we calculate? I know that somebody asked you this before but I could not understand your answer on another thread.