Sorry, I don’t understand what you’re trying to say. The math in your example is correct; hexadecimal 0x19 equals decimal 25, and 0x28 is decimal 40. But I’ve no idea why you’re posting that. To debug we need the payload and the expected result; please don’t post only half of the details each time!
That said, surely the following is wrong, according to the documentation you linked to, and according to the very different Node-RED code I found:
This basically is the same as:
Param.humidity = (Payload[3] * 256 + Payload[2])/100;
Param.temperature = (Payload[5] * 256 + Payload[4])/100;