bitcoin core – How do I calculate transaction charge for name to createpsbt()?

When my person invokes my app, he passes as an enter parameter the specified worth of the TX charge in satoshis per byte. I create a PSBT by calling createpsbt(). I exert full management over the dedication of inputs and outputs. For instance, I do not name walletcreatefundedpsbt(), as a result of I do not need bitcoind to pick inputs on my behalf. I’ve a hen and egg downside in that I must name createpsbt() in an effort to decide the dimensions of the TX in bytes, however I must specify the charge quantity earlier than the decision to createpsbt(). The very best resolution that I can consider is to execute a dummy name to createpsbt() utilizing a dummy charge worth, then measure the dimensions of the TX, then do an actual name to createpsbt() after appropriately calculating the charge quantity. Is there a greater approach?

Edit: I wish to spend all UTXOs. I need two outputs, one to obtain a hard and fast quantity, the opposite to obtain change much less the charge. I’ve tried many incantations, I am unable to get it to work. Here is my newest try:

raw_psbt = rpc.walletcreatefundedpsbt(
    # all UTXOs, as returned by listunspent():
    inputs,
    # one recipient: [ { addr : amount } ]
    outputs,
    0,  # locktime
    {
        "replaceable" : True,
        # I would love for the change much less the charge to go right here:
        "changeAddress" : xxx,
        "includeWatching" : True,
        "feeRate" : 0.005,
    }
)

This fails with Signing transaction failed (-4). Any concept what I am doing fallacious?

Edit #2: I am unable to use walletcreatefundedpsbt() as steered under to create the PSBT, as a result of that perform requires that its inputs be “solvable”, which isn’t the case in my surroundings. Extra data at this hyperlink:

Errors with walletcreatefundedpsbt & non-solvable UTXOs

So my unique query stays unresolved at this level.

Edit #3: So I requested on Slack. The issue is that our UTXOs are 1) not “solvable” and a couple of) can’t be expressed within the descriptor language, so Bitcoin Core has no solution to calculate the TX dimension earlier than the TX is signed. Apparently I must calculated it manually as defined within the connected screenshot.