You forgot some things, as Ron Gratz has in his formula.
Initially you are rear high. That first 100 lbs will not remove as much as formula says. Untill you squat to level, formula is on higher than actual. Once you go tail low, the formula is not taking enough off the FA.
Also, depending upon you rear spring capacity, 350 lbs will effect the springs on my GM 1500, assuming same WB, more than a typical 25, which is effected more than a SW 3500, assuming different spring packs, which is effected more than a de 35, which is effected more than my class 6 Navistar
My equipment trailer with 1500-1600 kids of HW, took 300-400 lbs off the FA of my SW 3500 with 6400 lb rear springs. I only lost 200 when I changed out springs to 8500, same loss as my 05 DW at 200-300 lbs. My Navistar with a 16500 spring pack, loses 60-100 lbs.
My TT with 600-700 lbs of HW list all of 60-100 lbs with the TT with 6400 lb springs.
So in a simple nutshell, no I have not gotten similar numbers for reasons given above in reality.
Marty