You may manually compute it by checking all of its children transaction's "absolute fee" (not fee rate, use a Blockexporer)
Or the lazy method of just going for small additions until you reach the point that it works.
Did you tried it and it worked? It is not working as I tried it up to 600 sat/vbyte. Just one child transaction with 1 input and 2 outputs.Or the lazy method of just going for small additions until you reach the point that it works.
Here's a test (testnet4):
- I've sent a sample parent @1.43sat/vb (200sat fee) to be bumped in the next step.
Link: mempool.space/testnet4/tx/aa70c7c824294dbee03e5012669406be1bcf273a95511085b328bd7654bddcae - Next, I spent its output with a child transaction @28.4 sat/vB (3100sat)
So to bump the transaction above, it'll have to add 3100sat on top of its own fee and the new transaction's minimum fee.
Since it was replaced in my test, I've secured a screenshot of that to-be-evicted transaction:
- Now for the test, I've bumped the parent transaction's fee to 3525sat which is 3sat more than what is required by the RBF rule. (3100 + 200 + 222 = 3522)
Link: mempool.space/testnet4/tx/5da5e1fb6730edcef66a8a1b0310371b5de00f52367a6ffc5f1836906a1023b8 - With it (the parent) being replaced, the child is now evicted from mempools that relayed the replacement parent transaction.
Link: mempool.space/testnet4/tx/a0b1ac2fe240ff230f23fd26f6f5e988cfab6652960d1bf95fd4783cb2d94565
That's how RBF rules work, if it doesn't work at your end, it may have been to your mention of fee rate since the addition should be based from the child's "absolute fee".
The "Preserve Payment" has nothing to do with the rule, it's just how the user's preference on how to add the required transaction fee.
If it's prevented by that option due to lacking change or other UTXO, he will not get to the point to see that error that he shared.