HTTP How to Calculate Content-Length multipart/form-data Including Binary

I'm working with a very obstinate API with arguably horrendous documentation (which is often also incorrect). I'm fairly certain my Cloud Flow is failing to POST a request to it containing multipart/form-data because I'm not passing the Content-Length header, at least that's the only thing I can deduce comparing the trace output of a curl command that is able to make the request vs what I have in my HTTP action.

I've resorted to using the smallest jpg that I think is possible (107 bytes) to try to reduce the potential for error, however I still haven't found a way to accurately calculate what the Content-Length actually would be.

For context, the file is uploaded to a sharepoint site, and then needs to be sent to a third party API as binary. So the reference to "Body" inside the body is actually the output from a Sharepoint Get File Contents action that happens before the API request. Also the current Content-Length value is being set by a Compose step I've been using to try to calculate it.

Does anyone know how to calculate what the body would be including the binary that's in there?

BTW, this one is close, but I'm not sending it as Base64

https://community.powerplatform.com/forums/thread/details/?threadid=3724db2d-b006-469e-9ef1-6fd741a01e66

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