The smaller the tractor, the more weight it needs added to safely use the loader. There are posts here and there about people never having to add a pound to do something but generally they are talking about much larger tractors which weigh more relative to their loader capacity to start with (especially old loaders). I also think it's the case that as tractors grow, they typically don't grow the distance between their front wheels and the bucket, as fast as they grow from the front wheels to the back end of the tractor. And in either case the BX is in the unlucky position that it carries its bucket VERY far from the front wheels, proportionally, compared to a lot of other tractors.
So basically, more is better. I would not worry too much about the exact number and would just build/use a box to carry rock or sand and fill it until the 3pt starts to not want to lift at idle, then unload a little and that's that. I'd actually spend more time making sure the removable 3pt weight solution you create is not a tipping hazard when its NOT on the tractor, because anything in the hundreds of lbs that a person can easily knock over on accident, is an accident waiting to happen.
My .02.
So basically, more is better. I would not worry too much about the exact number and would just build/use a box to carry rock or sand and fill it until the 3pt starts to not want to lift at idle, then unload a little and that's that. I'd actually spend more time making sure the removable 3pt weight solution you create is not a tipping hazard when its NOT on the tractor, because anything in the hundreds of lbs that a person can easily knock over on accident, is an accident waiting to happen.
My .02.