My electrum History tab shows 1.286 mBtc. But when I choose 'Max; on the send tab, I only get .80379?
It's quite complicated; the computed value of "
max" is based from the last fee rate slider that you've set.
So, if you've previously set it to an expensive value, it'll automatically consider the fee rate in that range to your maximum send amount.
'Fee rate' is 'sat/vB' so the to-be-deducted 'fee' is also based from the computed size (
vBytes) of your transaction (
can be seen in the next window).
Pro tip: If you're on Desktop, after clicking "
Max", hover your mouse over that button and click on it again until a tooltip shows up.
It'll show you the fee that was deducted from your total amount, that should answer this topic's question.
Of course, that's not final since you can adjust that after clicking "
Pay..." and/or "
Preview" in the next window.