Assuming the mempool high fee rate priority is 50 sat/vbyte
The parent transaction paid 30 sat/vbyte
The child paid 70 sat/vbyte or higher
Note that your example will only hold true if both parent and child transaction are the exact same size, which is often not the case.The parent transaction paid 30 sat/vbyte
The child paid 70 sat/vbyte or higher
To work out what fee you need to use for the child transaction, you first have to calculate the size your child transaction will be, add that to the size of your parent transaction, multiply by the final fee rate you want to achieve, subtract the fee the parent has already paid, and then use that fee for your child.
So let's say your parent transaction is 500 vbytes, paying 10 sats/vbyte. Your child transaction will be 200 vbytes, and you want the final fee to be 20 sats/vbyte. So your total transaction size will be 700 vbytes, your total fee will be 700*20 = 14,000 sats, you've already paid 5,000 sats, so your child transaction needs to pay 9,000 sats. This will work out to 45 sats/vbyte for your child.
This obviously becomes more complicated when you start considering multiple unconfirmed parents, bringing in additional inputs to the child transaction, and so forth.
