NOTE: if you start by giving the calculator your *desired* frequency of operation, the inverted V - when installed in its final position - may be still end up too short or too long, depending on the environmental conditions mentioned above.
As the two sections of the dipole are lowered below the feed point, the angle between the two legs decreases: The angle below the two sections of a horizontal dipole is 180 degrees.
In the case of the inverted V we must add - to the list of environmental variables influencing the half-wave dipole - the angle between the two legs of the inverted V. It is adjusted to take into account the special characteristics of the inverted V. The formula used by the calculator to compute the wire lengths for the inverted V is based on the formula for a half-wave dipole. Will give you some leeway to trim the wire ends back if the inverted V resonates too low for your purpose when installed in its final position. Than that of a dipole at the same desired frequency of operation. Calculator above will give you a wire length for the inverted 'V' which will be about 4% shorter