This is a very interesting analysis of the difference between the popular vote and electoral college vote.
If Biden gets just 1% more popular vote than Trump, he has only a 6% chance of winning the electoral college. In fact anything up to a 3% lead sees Trump more likely to win.
For Biden to be confident of victory he really needs to be aiming at a 4%+ lead in the popular vote. His current lead in 7.5% so only needs a 3.5% slippage for the race to get competitive.