I use this LFO technique all the time myself 
I have been thinking of creating a tool to help with this kinda stuff, but I could put together this mockup for you in a few minutes if you want.
In the meantime, a nice tip is to build your LFO envelopes in the Automation editor instead. Simply enable automation on the LFO frequency, then you can type in the exact frequency values for each point. When you’re done, just copy the envelope points from the Automation editor right into the custom LFO envelope.
Edit: I’d personally like to see the overall functionality of envelopes expanded into something a bit more like this → Envelope Editor View
But using the Automation view as a temporary LFO workspace is an OK-ish workaround for the moment.