Inflight mental arithmetics

[D O Guerrero]

I'd just do 30/250 x 60 gives about 7 minutes for the descent (or doing it in my head, 25 miles in 6 minutes, plus 1.25 gives about 7 mins).
10/7 is roughly 1.4 so 1400 fpm would seem suitable.
I don't see any need to make it more complex!