Calculated Field from Array

I'm attempting to create a new mapping in an array using a calculation between to existing mappings. Example: '(space/total_space)' => [ 'key' => 'Disk_Free_Percentage', ], Where 'space' is divided by 'total_space' to get a percentage. The code I am using this far is below and I am stuck on this last piece. Any help or pointers would be greatly appreciated. // Call API to get servers $rawData = $admin->servers(); // Data mapping $map = [ 'server_name' => [ 'key' => 'Node', ], 'space' => [ 'key' => 'Disk_Free_GB', ], 'total_space' => [ 'key' => 'Total_Disk_GB', ], 'ram' => [ 'key' => 'Free_RAM_GB', 'normalize' => function ($data) { return number_format($data / 1024, 2); }, ], 'total_ram' => [ 'key' => 'Total_RAM_GB', 'normalize' => function ($data) { return number_format($data / 1024, 2); }, ], 'numvps' => [ 'key' => 'VMs', ], 'version' => [ 'key' => 'Version', ], ]; // Remap data $data = []; foreach ($rawData['servs'] as $row) { $item = []; foreach ($map as $fieldName => $field) { // Validate key exists if (!isset($field['key'])) { throw new LogicException(sprintf('Field %s must supply a key', $fieldName)); } // Get data from API if (isset($row[$fieldName])) { $value = $row[$fieldName]; } else { // Use null if data not given by API $value = null; } // Run normalize where set if (isset($field['normalize']) && is_callable($field['normalize'])) { $value = call_user_func($field['normalize'], $value); } // Map to new data $item[$field['key']] = $value; } $data[] = $item; } // encode JSON $json = json_encode($data)