A simple rule of thumb is.
Climb rate = Gradient (%) x Groundspeed.
So for example if you want to achieve a 5% gradient at 180kts G/S you need to climb at 900 fpm.
Another one, if you need to change your altitude by xxx feet/mile multiply that by your speed in miles per minute and the answer is feet per minute.
feet/mile x miles/minute = feet/minute. Up or down.
e.g. you want to lose 3000' in 10nms or 300ft/nm at 180kts (3nm/minute) descend at 900fpm.
Is this any help?
Regards,
BH.