The back of the Jepp CR2 computer gives rough descent rates from groundspeed...set the TAS index at the groundspeed, find the 3deg mark (for a nominal 3deg descent path), on the inner scale and read the rate of descent on the outer. 250kts GS = 1300fpm. If you don't have a Jepp CR2 computer, you should.
Quick method...GS/2 x 10 = descent rate for a 3deg path.
PJ2