The higher your body fat the lower your test gets and the more insensitive to insulin you become, making your calorie partioning worse.
True, but the lower your body fat level gets below your set point, the worse your calorie partitioning becomes.
I think you need to clarify here MJR - you're talking about partitioning at low bodyfat but as you drop bodyfat, you're not bulking are you? That happensin a caloric deficit, certainly once you're dipping into the single-digits for bodyfat %.
You should ideally stay somewhere in between, say 10-15% for males. or 8-12% if you need to compete. For some endomorhpic people it may be higher, for some ectomorphic people it may be lower, but that is around ideal for most.
You and I may be in agreement - I suspect you just aren't being very clear.
Dieted-down lean people can and do partition badly when they switch over to bulking unless they ease into it.
This isn't the same as a lean person who adds a little extra to his caloric intake and starts "riding the wave" - the mythical "clean bulk" where most of the weight gained is muscle. This is a very slow way to gain muscle but the perk is fewer wardrobe changes - you just gradually get bigger.
For most, bulking will go well until, as you and sensamilia suggest, bodyfat levels get too high and partitioning goes to shit.
Biggly may not be posting much so I'm not sure if he'll see this but to answer his question, "it depends". It depends on how fast you gain and how well you partition. How fast you gain depends on how much of a caloric surplus you run. How well you partition depends on your training, your bodyfat level and your genetics - as well as what type of assistance (if any) you're running.
In terms of actual poundages, most men can probably count on 1-2 lbs of lean mass gain a month, with about the same amount of fat-gain. A 500-calorie a week surplus will provide sufficient calories to support a 4-lb a month fat-gain, supposing ALL the excess weight gained is bodyfat. My suggestion would be to decide how much fat you are willing to gain per month as a max, and how long you're willing to run this experiment before cutting. Do those and you won't turn into a monstrous fatass by the time you're ready to switch gears and unveil the fruits of your labour.
My .02