How to convert pixel coordinates in 1/72 inches?

[stasmotorny]

Hi, guys. I am working on documents signature implementation. It have few steps:

1. User see PDF file and should tap on the place where signature should be placed. On this step I'm using https://github.com/wonday/react-native-pdf this library to display document and take tap's coordinates.
2. After step 1 I navigate user to the screen where he should draw his signature.
3. After user draw signature I'm using pdf-lib drawImage() method to sign document.

There is few problems in this approach:

1. 0, 0 coordinates from react-native-pdf are in top left corner and as I understand coordinates are in pixels.
2. pdf-lib as I found out is using 1/72 inch as measurements units and 0, 0 point is in bottom left corner.

As result I can sign the document but with wrong placement of signature. So I need some way to properly convert coordinates taken from react-native-pdf to coordinates that are used by pdf-lib to place signature.
I already tried a lot of variants but no one works fine. In current implementation I'm using formula like this:
(pageHeight - signatureCoords.y) / (160 * PixelRatio.get()) / 72
So maybe someone can give me an advice how to properly place signature in PDF?