As you noted you have the direct odds to make a call so a required future bet for implied odds need not be calcaulated.
Anyway, if you did the arithmetic correctly, it still works out though the concept of a negative future bet is meaningless.
The equation for future bet in this example is FB$ = 40*(65.8/34.2 - 80/40) which is -3.04, not -3.52 as you have. The corresponding meaningless EV equation is
EV= 0.342*(80 - 3.04) - (1-0.342)*(40 ) = 0
I guess this shows that even meaningless math can work out