[sesqu]

This is exactly how I do it in my head. I was never taught to, but found myself multiplying multifigure numbers in my head often enough to come up with the gridding system. It becomes laborious after five figures or so. I was never that fond of the long forms, and didn't use them after they stopped appearing in the exams.

Incidentally, I didn't bother with a calculator until 12th grade. Since then, I've started making more mistakes, and now have to do any mental calculations several times over.