WEIGHT FOR IT !!

For me, rule of thumb is pack as much weight on the rear as possible. If I had to add...say....110g in the rear and couldn't divide it equally (front and rear of rear axle), then i would put the heavier amount in front of rear axle, i.e. 60g/50g. As long as there is any kind of weight on DFW, it's never too light. I don't want to lighten the DFW by putting the extra (heavier) weight rear of rear axle because there is virtually no increase in potential energy and it will decrease weight on DFW, thus, expose unnecessary risk to stability. Perfect rear weight location would be center line of rear axles. This will provide max potential energy without any unnecessary risk to light weight on DFW.

Without divulging any top secrets, what is absolutely too light on the DFW?

Now THAT is good info. Thanks for sharing Dan!

A 99.5% percent 60g tungsten bar will take up the same amount of space as 12 cubes but will weigh the equivalent of 13 cubes (about 4.7-8g more). I place this bar in front of rear axle and install 90-95% of the weight needed behind. This will leave a space in back to top off after the car is completed.
Bias weighting: I try and make the rear weight on both wheels as even as possible. Much of this is achieved by sliding the weight to the dominant side during a build. You can position the weight to be even with side of car making the weight part of the body structure. The alternative is to place additional weight on dominate side forward of the weight already in front of rear axles. However...now your getting further away from center of rear axles where that additional weight is not as efficient.
I have older cars where there is a 15g or so weight diffrence between both rears. Since I started paying attention to getting them even, my cars have been posting faster times but I cannot show proof that perfect balance correlates to faster times because there were other things I was doing that likely contributed. It would take a tightly controlled test plan to see how much speed increase is associated with a balanced rear and a rear that is 15g or so off balance. I go for even balance because an off balance configuration places more load (friction) on one of the two wheels/axles, thus, creating the weak link between the two and limiting the speed cieling to the side with more friction. This is one of those elements that physics supports, so I just follow logic without actually performing an isolated test to verify. It's worth noting that some of my cars that are not close to an even balance still perform well but again, we are fighting for every .0001 of a second we can find.