Help with a forecast function.

I am trying to figure out how to calculate the number value to increase my
percentage from whatever it is currently to 95%. I have 3 pieces of data,
good records, nuetral records, and bad records. In cell 1a I have the % of
cells b1, c1, d1. Here is my calc:

=(b1/SUM(b1:d1)

a1 b1 c1 d1
87% 20 2 1

I am trying to figure out how many more of B1 I need to reach a goal of 95%
in cell a1. My answer in this case is 32 more, which I want to display in
cell e1.

How can I do this?

TIA,
Bob W.