I've realised I have an architectural issue with @v4vapp which I don't need everyone to understand but I need to change.

I need to move the part that does the price conversion out from being inside my private code into the code that runs on a public API.

The upshot of doing that would be that I could give an exact Lighting invoice and then guarantee a certain number of Hive or HBD would be returned if paid in the next 10 minutes.

This is a bit of work but it is essential so I will start doing it though I'm going to be tied up for the next 3 or 4 weeks. Once its done it will make everything you're trying to do here much simpler.