Venmo charges business transaction fee of .10 and 1.9%. Let's say I want to know how much to charge to ensure I get an accurate and specific take after Venmo's fees. What would the formula be?
Right now, it's a matter of brute force; I guesstimate and do the math. But as you add money to offset, the percentage Venmo takes increases. That's why I can't figure out how to do this accurately from an input of my desired take without brute force. Forgive me if I'm missing something obvious.
If I understand your question correctly, Venmo takes $\$0.10$ plus $1.9\%$ of every transaction. We want to make $\$d$ from a transaction, the amount of which is $\$t$. Venmo will take
\begin{equation*} \$0.10 + 0.019t \end{equation*}
as a fee, and so in order to make $\$d$ from the transaction, we need the transaction amount to satisfy
\begin{equation*} \$ t - (\$0.10 + 0.019t) = \$d\implies \$t = \frac{\$d + \$0.10}{0.981}. \end{equation*}