Deep sixes work fine, you just have to figure out how-many-pulls-is-how-deep (an inexact exercise at best). Once you dangle on a fish remember how many pulls you let out and repeat as necessary. If you really want to know: The size of the six, boat speed, current direction, flasher/dodger, all have an effect on how far down the thing will go in my experience (Oh yea and seaweed!). Remember geometry? It's basically about triangles when you estimate/guess depth with a six . (Test this by letting out fifty pulls in forty feet of water, if you don't snag bottom you aren't down fourty with fifty pulls! If you are, bye bye deep six) I used one for a considerable time and eventually was able to get a relative idea of how deep I was going. Just hang the rig off the side of your boat on a sunny day and tool up to trolling speed with five, then ten, then twenty (it'll disappear about here) pulls, look at the line angles and you get a close approximation of how much of the triangle is taken up in aft movement (elongating of one leg of the stern/line/depth triangle) Cornfused yet? just try it and you will see what I mean. You get an 'eyeball' idea of how deep different pulls off the reel go...based on the line angle.
If at all possible get a 'rigger if you want a closer approximation. Even with a rigger it's still got aft drift, elongation on one leg of the triangle and subsequent 'shortening' of the metered depth (on the counter). My one cents worth.