How much of battery weight transfers to tongue?

It's proportional, if the load is halfway between the axle and coupler it's 50/50, if it's 3/4 of the way it's 75/25. In this case it's probably close to 95/5. (for purposes of this discussion I did not want to muddy the waters and simply disregarded the normal 12 percent tongue load). Sufice it to say you will indeed save nearly 100 percent of the difference of the battery(s) weights in tongue weight.