MY EASY WEIGHT RULE
GRUBS and shads can be quite heavy, but in water they are almost neutrally buoyant and need a weighted jig head to get them down to the right depth.
My rule of thumb is to use one gram per foot of water.
So if I want the lure to run at 5ft I use a 5g jig head. For 15ft I increase this to 15g (below) and so on.