Some guys use a 100' rule. 20' down 80' back, 50' down 50' back, 80' down 20' back. You get the drift.
I like to put my spoons back about 20-30' unless I'm in shallow water, then they go back more.
Flasher/fly are always about 8-10' back. Plugs go 10-20' back.
Flasher/fly works very good when the fish are deep down.