I would characterize anything under 55% a squeaker. Anything between 55% and 60% a "medicore win" and 60% to 65% a "convincing win".
However, since that only applies to the Electoral College, it doesn't say too much. A few thousand votes can make the difference between winning with 290 or 350 electoral votes. Even in past landslides; if you peel off about 2-3% from FDR in each state within single digits, his 432 electoral vote (81%) landslide gets reduced significantly. He won several battlegrounds, even New York, by small margins that year.
Not to mention third parties. For example, Ronald Reagan had three electoral votes more than Lyndon Johnson in 1964 (489 and 486), but he just won 50.7% of the popular vote and a margin of nine points. LBJ won 61.1% and beat Goldwater by over 22% with the almost the same number of electoral votes. Even Joe Biden won a higher share of the popular vote this year. He also won a higher vote share than Bill Clinton in 1996 and Barack Obama in 2012.
Yah in 1980 if you take 3 points of Reagan's final margin this is how the map would look like
Reagan wins 369 EV instead of 489
Yeah, though the Electoral College in 1964 was actually underwhelming compared to the popular vote. LBJ should at least have won Georgia that year.
What's interesting about 1980 is that if Carter takes all states Reagan won with a plurality, Carter would have won (I think it was 281 electoral votes). However, that's just in theory. Reagan would have won a head-to-head matchup without Anderson quite easily. Carter was DOA in 1980 against any credible challenger.
