Thanks for the tip - I’ll practice doing it that way.
From 100, 93 is easy. Then I figure it takes 3 more to get to 90, and I need another 4, so...86. From there, 6 to get to 80, plus one more for 79. From there, 72 is easy. And so on. But it’s not hard to get lost in the process when being asked to “perform” and to prove you’re not getting senile.
Similar to your method is a shortcut pilots use to figure reciprocal headings. You need to just add or subtract 180° from the heading to find it’s reciprocal. For instance, 040° + 180° = 220°, it’s reciprocal*. But adding or subtracting 180 to a number in your head can be hard, especially when tired or under stress. The shortcut that makes it easier is to either add 200° and then subtract 20°, or subtract 200° and then add 20° back in. Adds a step, but most folks find it a whole lot easier using that hint.
*Runways are numbered according to their magnetic bearing, rounded to the nearest 10°. So Runway 4 (roughly 040° magnetic) will be Runway 22 going the other way.