SetMargin With RectTransform - Stretch

Hello.

I have a problem with the size calculation of setMargin().
I looked for other issues that fit the size of the web view to RectTransform, but the difference is that I use Stretch (Ancher Preset).
Currently, I've succeeded in roughly sizing it, but I'm having trouble fitting it completely, and I don't know how to calculate the size.
I am attaching my code and a capture of the current situation. I'd appreciate your help.

int marginLeft = (int)_BG.offsetMin.x;                      // driven by RectTransform Property
int marginRight = Mathf.Abs((int)_BG.offsetMax.x);
int marginTop = Mathf.Abs((int)_BG.offsetMax.y);
int marginBottom = (int)_BG.offsetMin.y;

int screenWidth = Display.main.systemWidth;
int screenHeight = Display.main.systemHeight;

int left = marginLeft * screenWidth / 640;                     // 640, 1148 driven by Canvas Size
int top = marginTop * screenHeight / 1138;
int right = marginRight * screenWidth / 640;
int bottom = marginBottom * screenHeight / 1138;

Debug.Log($"{left} {top} {right} {bottom}");

_webview.SetMargins(left, top, right, bottom);

No repository file or test is named. Start by reviewing the attached Unity RectTransform Stretch setup and the margin calculation shown in the issue, then trace the _webview.SetMargins call; done means the web view fits the stretched RectTransform completely across the reported screen and canvas sizes.