I wrote a fairly simple bash script to help figure out how much to bump your child fee in mempool.

If someone can find a formula to do this based on just the change and not the child script that would be great. You can also clone it here https://github.com/jtymoszczuk/cpfp-calc/blob/main/cpfp-calc.sh

#!/bin/bash

echo "This estimates how many sats/vbyte you need to bump your child tx to get your desired parent fee"

#x= targetChildFee #x = (sat/vB_effective * (vB_parent + vB_child) - current_parent_fee) / vB_child

read -p "Enter the total amount of sats your current(parent) tx cost? " currentFee

read -p "What fee do you want your tx to be in sats/vbyte? " effectiveFee

read -p "Enter Virtual size of Parent? " vBParent

read -p "Enter Virtual size of child? " vBChild

x=$(echo "scale=2; (($effectiveFee * ($vBParent + $vBChild)) - $currentFee) / $vBChild" | bc)

echo -e "Your Child tx needs to be \033[32m$x\033[0m sats/vByte for your parent tx to be \033[32m$effectiveFee\033[0m sats/vByte"