'x' is the number of sales with positive feedback you need.

You have

You need

With a little algebra, you could solve that for 'x' and pick the next higher integer. You can't have half a feedback, right.

HOWEVER, it may be MUCH easier to do it this way.

1 - 0.998 = 0.002

You have

You need

Either way. Again, watch for that partial feedback. Move up to the nearest integer.