if you know the x and y coordinates of the point you want your arrow to point at you just do
let your_x = ...
let your_y = ...
let target_x = ...
let target_y = ...
// relative to your character
let x = target_x - your_x
let y = target_y - your_y
// calculate distance with pythagoras
let distance = sqrt(pow(x, 2) + pow(y, 2))
// get the relative distance into a range of 0 to 1 or 0% to 100%
let percent_x = x / distance
let percent_y = y / distance
let arrow_on_screen_x = screen.width / 2 + percent_x * screen.width / 2
let arrow_on_screen_y = screen.height / 2 + percent_y * screen.heigh / 2