BackFire
Blood. It's nature's lube
You can already make some estimates about overall turnout which is a big key.
We know that 100 million+ people will have voted early by tuesday. And from what I've read estimates are somewhere between 50 million to 70 million people will vote on election day.
If those numbers hold true, and the polling is accurate as far as the percentage of the advantage Biden has with mail in voting, and the advantage Trump has as far as election day voting (according to polls both are pretty equal), then Biden will have the advantage as there will be more mail in voting done this year than election day voting. So on its face, Biden has the advantage and if you just compare those two numbers Biden should have a 10 million+ vote advantage.
Of course there are various wild cards like how many mail in votes will get thrown out (many get thrown out for various reasons every election), how many people actually show up on election day - there could be an election day voting surge that gives Trump an advantage or more people could show up on election day voting for Biden than is expected - or the advantage that each candidate has in their respective voting group (early for Biden vs Day of voting for Trump) could be wrong. So lots of variables still.