yeah that's what I am doing now, and still overwatering... or so it would seem... it's really hard to find that balance if you're not there looking at it everyday...
It takes some experimenting before you get it right. And even then it isn't exact. Plants go through more water and certain points in their life cycle so you never get it quite right - you can only hope to get close.
I see what you are hoping for with a sensor, but I'm of the opinion that simpler is mo' better when it comes to automation. It would take very little for that sensor to be a tiny bit off and you empty your reservoir in one watering trying to reach some set point and you come home to dead plants. I like the simplicity of a timer on/off.
Quit watering your plants by hand and just use the timer method and you'll get it dialed in pretty close for when you are away and it counts for real.
Like so many things, the key is practice.