That formua per the email and per B31.3 is for a blind flange which is supported either side by line flanges not an blind end flange which is what the stock blind flange is and which is designed as a flat plate to a different formula refer ASME V111 div 1 which is why it is as thick as it is..if you use the ASME formula for flat plates for your pressure you will find it is close to the stock flange thickness. I would worry more about the HDPE pipe, it will be temperature limited and the weaker link in your system by the sound of it.