Unstable is a relative term... It may be stable under most normal towing conditions, but, what about emergency maneuvers? That is what the 12-16% of GVW on the tongue is meant to provide. Less than that is considered light enough to be "unstable" under some conditions. Adding weight behind the axle is going to lighten the loading on the tongue; that's a fact of physics. If your tongue weight is still in the acceptable band of percentage of GVW, then you are good to go, if it isn't, then it is considered "unstable" no matter how many miles you may have managed to tow the rig. Another physics fact is the amount can only be determined by knowing the distances involved in the fulcrum lengths and the exact placement distance on those legs of the fulcrum. So, "rule of thumb"? Nope. The only "rule of thumb" I've ever heard bandied about is that if you placed weight in the trailer, directly over the axles, then in that theory, 80% would go on the trailer's axles with 20% added to the tongue, but, again, that isn't a hard and fast rule because the fulcrum point isn't actually a "point", but, is spread across the distance of the axle to axle center points and introduces more mathematics to the equation. I've never seen any math the supports even that rule of thumb, because logic dictates that if you placed that weight directly over the fulcrum point it would have no appreciable effect on the tongue weight as all the weight would be born on the axles; perfectly balanced. Any shifting of that weight, based on distance from the balance point is going to create a sliding scale of positive and negative numbers regarding the effect on tongue weight. There is no "rule of thumb" to be used; only actual weight readings taken on scales can tell you. Even the big rigs know that the only way they can be sure of a safe load is to weigh the rig and that's how the CAT scale company stays in business. Part of the reason why big rigs trailer wheels are at the end of the trailer is so that they don't have to worry about load balance; all they have to worry about is gross weight since, even as the trailer is emptied, the 5th wheel is designed to support more than half the gross weight. So sorry, there is no method of reasoning away a weight or balance worry short of actual measurement of the conditions present on your trailer...and it has to be yours. Even "similar" can be a wide range of variables. How many canned goods to each person carry? Where are those canned goods? How much water and where is located? What other shifting loads might there be? If you both carried bottled water in a forward storage bay, are they both the exact same distance from the tongue?
There isn't a "feel good" rule of thumb for weight. You either trust your gut and go with it, or, you haul your "normal" rig to the scales to find out the truth.
I'm pretty sure that's what you asked the first time.
