Calculating Percentage need to hit new percentage

If they have attached 2 out of 10, I assume the remaining 8 were not attached and never will be.

How many new attachment attempts will there be? That bit of data is required for this calculation. Here is why.

To continue your example, if they have 10 more attempts, then they will need to attach 6 out of those 10, or 60%, to reach a total of 40%, which is 8 for all 20 attempts. But if they have 20 more attempts, they will need to attach 10 of those, or 50%, or reach a total of 40%, which is 12 for all 30.

What in the world is an "attachment"?

You have to know one of two things:

How many attempts will be included in the total

or

You assume that subsequent attempts will be 100%

In my first post I described why the first number would be needed. Let's take a second scenario: The employee has 2 of 10 for 20%. How many more do they have to do, at a rate of 100%, for the overall rate to be 40%? I think that is what you are saying by "a way that will add 1 to their attached number and attempts" but if you provide an example of the final result you want, it would go a long way.

For the second scenario, let's say A1 has 2, A2 has 10. You don't need the resulting percentage for the calculation. Assume B1 has 40%, the target. To determine how many more they need for 40%:

=CEILING.MATH((A1-B1*A2)/(B1-1))

gives the next whole number that will give 40% or better.

ETA: In this example they need 4 out of 4 to reach 40% (and they will be a little better than 40%)