Most everyone knows that if you have

A square ending in 5 ala, 25, 125, 35 1205

Generalizing this when you have the last digit of a number summing to 10 that you can simply do the same 21*29, 37*33. (If you have 2 digit squares memorized then you obviously just subtract from the square and save yourself further calculation by using the fact ( I generally see 21 * 29 immediately as 625 - (8/2)^2 and struggle with the complement to get 609, but ignore that for the moment)

Now the obvious but fun part I haven’t seen tossed out before; 23 * 37

Yup… you can still quickly use the symmetry, numbers gotta love em.

Of course, you will point out that if have your squares nailed then

900-1=899 $ lol.

and even if the number was

21 * 39

then you are still centred on 30 and

900 - 81 = 819

but 34 * 76 might feel like a bit of a pain in the butt so it is nice to have a couple of choices when you see this. ( although 55^2 - 21^2 isn’t that hard )