What's your "Napkin Math"?

[RayS]

I've been flying the B1900 now since 2015 and have developed a small stack of napkins when I fly.

One of my favorites is the Top Of Descent Napkin calculator...

Given the following:
FL:      250
IAS:     250Kts
VS:     -1000 (I always descend at this rate)
Tgt Alt: 0

In  the Sporty's TOD Calc in my phone's E6B, this formula will give me a TOD of roughly 104nm out.

 I usually round it to 100nm.

I use the numbers all the time and it's proven quite handy.

Interpolation is easy. If I'm at 13,000, TOD will be around 50NM, 6000ft: 25NM...

Would love to hear from the crowd if y'all have any mental napkin math....