Experimenting with UTXO Split Size and learning that I still have more to learn

Right now there are two key things I'm trying to learn about, both of which I think should be able to help me optimize my Runes minting costs. What I have been experimenting with the most lately is UTXO split size. Before Runes launched the advice I got was to make my UTXO splits 250,000 sats. I did this, and it worked great on mint weekend when fees were over 100 sats/vByte.

Now, fees are a lot lower, here's a look at what I'm seeing this morning:

I couple weekends ago I experimented with UTXO split size and went as low as 23,000 sats. My logic was, if my full transaction cost is coming in at under 10,000 sats total, then I should only need a little more than that in my UTXO split. So I expected 23,000 sats to be more than enough.

It turned out I was wrong. My transaction cost (for minting Runes) went from about $5/mint to $10/mint. This makes me think that I must be using two UTXOs rather than one, for each transaction...but I don't know enough about how to look under-the-hood to verify if that's happening or not.

What I do know is that optimizing the size of my UTXO splits will allow me essentially to have the most quarters possible to put in the minting machine. Right now, if I have 1,000,000 sats to split, I can split into four UTXOs of 250,000 sats each, but then I can only perform four mint transactions at once. Since those mint transactions won't use 250,000 sats each, I'll then have to wait for the block to clear, get my change, and then mint again with somewhat arbitrary UTXO split sizes which will be determined by the change I got back.

This feels inefficient to me. It seems like it would be better to know exactly what size UTXO split I should make, based on the average transaction cost at that time...but it's clear I'm missing something since my attempts to do that so far have resulted in doubling my mint fees.

So - as I'm learning every week, the more I learn about Bitcoin, the more I need to learn about Bitcoin. More to come, this weekend I'm going to really try to do a deep dive into this one!