What's the simplest way of calculating the optimum fee? For 'Satoshi per byte', is it possible to know how many bytes the transaction is likely to take?
yes, here is an in-depth breakdown of how transaction size is calculated:
https://bitcoin.stackexchange.com/a/92700this definitely isn't the simplest way though. a good wallet like electrum will show you the actual transaction size in bytes and allow you to choose the fee rate in satoshi/byte.
https://bitcoin.org/en/wallets/desktop/windows/electrum/I've tried Earn.com and the ones you mentioned before, and I found all of them to be overestimating fees. I suggest this one, it's far more accurate:
https://coinb.in/#feesi didn't know coinb.in had a fee estimator. it's a tad frustrating that they include bech32 as an option for inputs but not outputs. it makes a difference due to the segwit discount. the lowest possible recommended fee on coinb.in is 0.00000144 BTC but it should be closer to 0.00000110 BTC.
my preference is for Jochen Hoenicke's fee estimator. great visualization for estimating different priority times, ie next block vs within 8 hours, etc:
https://jochen-hoenicke.de/queue/#0,24h