For ROD for 2X profile try 8 X G/S. Quite easy to do in your head
All speeds in the 200 knot range will be 1600 fpm plus the extra for the amount above 200 knots.
All speeds on the 300 knot range will be 2400 fpm plus extra for speed above 300 knots.
e.g. 250 knots = 1600 + 8 X 50 = 1600 + 400 = 2000 fpm
It's not an exact calculation as 2 X descent rate for 300 knots is 2500 fpm. However close enough.