I don't think you need to worry about a dose to weight ratio. You have experience with gear and know how doses will effect you. Over time, you've obviously taken more, probably as you've seen results from older doses not working as well as they once did. If we were to find the medical formulas as to how much deca per lb of bodyweight, I think it wouldn't really apply to you, considering the dose scheme doesnt take into account that you have increased your doses over time. 

If I were to make any suggestion to you, it would be to focus less on the scale and more on how you look. I would never let myself get to 20% BF. Thats far too much work to take off when it comes to dieting. Don't get me wrong. There was a time that I'd put on any weight I could, but it always seemed to be counter productive in the end when I had to diet most of it off. Of course you want to top off your weight class, but most importantly, you want to LOOK like you top your weight class off. Follow me? If you are 20% right now at 235 lbs, that leaves you with 188 lbs of lean mass. Now of course you can't get rid of 100% of your body fat, so I'm sure you have more muscle now than you did at your last show, but it's no where close to the 50 extra Lbs you are carrying. The lean gains will come in time. Just keep plugging along and keeping things cleaner in the off season. Maybe insulin is the issue? I can't say without knowing your body, but I have seen it mess up a lot of great physiques.