Complex calculations in Gravity Forms may not work in the order that you'd expect. In this scenario, instead of subtracting the total value of the calculations from the value of field 15, we have to force the order of operation by subtracting it at the end, which creates a negative number. To return the number to positive, we multiply the total value by -1.
(
( {:15} * ( {:3} * 0.01 ) ) +
( {:15} * ( {:16} * 0.01 ) ) +
( {:15} * ( {:17} * 0.01 ) ) +
( {:15} * ( {:7} * 0.01 ) ) -
( {:15} )
) * -1