That's very much my use case though, all my expenses are on my calendar and I add them all once in a while - so the source is a browser tab, I put the equation

That's very much my use case though, all my expenses are on my calendar and I add them all once in a while - so the source is a browser tab, I put the equation in the address bar as I'm reading it inline, then hit enter when I'm done and navigate away. Using a local app would actually add one more step to that process.