Php 在同一数组中使用相同的键值

如何正确编写此代码？第四行，我想在同一个数组中使用前两个键的值

$pixel_percents = array(
    "complete"=>.5 * 768,
    "wip"=>round(.2 * 768, 0, PHP_ROUND_HALF_DOWN),
    "remain"=>$pixel_percents["complete"] - $pixel_percents["wip"]
);

以后再做吧

$pixel_percents = array(
    "complete"=>.5 * 768,
    "wip"=>round(.2 * 768, 0, PHP_ROUND_HALF_DOWN),
);
$pixel_percents['remain'] = $pixel_percents['complete'] - $pixel_percents['wip'];

以后再做吧

$pixel_percents = array(
    "complete"=>.5 * 768,
    "wip"=>round(.2 * 768, 0, PHP_ROUND_HALF_DOWN),
);
$pixel_percents['remain'] = $pixel_percents['complete'] - $pixel_percents['wip'];