checking the given example:
41200 * (8.2 - 1.245)  286546

41200 * 0.000003 + 1.245 == 1.3605 



(1.3 - 1.245 ) * 12400 == 682 
(12400 * .000003 ) + 1.245 == 1.28
682 / 1.28 == 532

so lowest resistor value must be 500 about...


(4.2 - 1.245) * 12400 == 36642
\ 1.28 == 28626 so 28700 (use 1%)

(6 - 1.245) * 12400 == 58962
/ 1.28 == 46064
EDIT: this should be let's say 6.9 as the float voltage. (13.8 / 2)
so
(6.9 - 1.245) * 12400 == 70122

/ 1.28 == 54782
so 54.9K (is the closest E series)