Help with budgeting formula

Hello. I'm digital ads manager and I'm working on standardizing a table on budgeting when a client give us their ad spend budget. They often give us a gross number when we need to figure out net spend, ie ad spend plus our service fee.

For example, if they tell us their gross spend is $5000, our service fee is 20% of ad spend, I'd like to have a quick way to figure out the net ad spend.

$5000 - 20% is clearly $4000/$1000, but $4000 + 20% is only $800.

In reality, $4,166.67 + 20% = $5,000.004 (which I had to figure out through trial and error on the calculator).

If cell A1 is GROSS $5000, what can I put in cell B1 to help find NET ad spend?

Thank you all so much.