Unfortunately I don't know of a tool that calculates it well how much you should pay for that new tx.
It's not difficult to do the calculation.
S
A = Size of the unconfirmed transaction
S
B = Size of the new transaction (the transaction you will make for doing CPFP)
f
A = The fee rate used for the unconfirmed transaction.
f
B = The fee rate you should use for the new transaction.
f = the fee rate required for a fast confirmation.
fB = (f*(SA+SB) - SAfA) / SB