finding the profitability threshold

[#1]

Hello,

I try to figure out how to calculate the number of subscription weeks to get the advertising investments profitable.

Materials I got is :

• gross costs (I'm able to drill down to daily basis for each support / campaign)
• subscription's fare is Z each week
• records of unsubscriptions over time for each subscription day of last year (see picture attached)
• those show a quite steady trend over time

My think is that :

• I need to determine the right formula for unsubcription rate ; something like "ax² + bx + c = 0 " where x is the number of week
• then for each week solve "ax² + bx + c = week gross cost "

Do you think my method is correct ? and can anyone help me make it works ?

Thanks a lot

Cheers

Julien