For you especially @Arunav_Mandal, it’s easy to google “Usable Capacity” for Tesla. I easily found one for Model 3 Performance as an example:
That means 1% = 0,76kWh – Theoretically, without wear and tear.
It’s best to measure your actual “coefficient” during summer. But since it’s winter, use what you find for your model online as a starting point. Let’s say you have your car set to stop at 80%.I would turn that setting off temporarily for this first test. Just in case the coefficient you are using all of a sudden is too high.
Now if your car is at 55%, take aim at for instance 90%. So 90–55 => 35% to charge. The higher the charging-% the better precision your output will have. Now you order 35*<your coefficient> kWh in Piggy. Then you check what temperatures is predicted during your charge. Let’s say it’s maximum 7° and at lowest 2°C.
When charging is done, you will see either you are spot on, below or over 90% battery. If it’s 100% the test is useless 
Let’s say you have 82% battery when charging is done.
You now know that you missed your target with (1-(82/90))*100 = 8,9%.
So at least for the temperature range 7°C–>2°C, you should assume you need to add 8,9% to your coefficient. You can make assumptions for lower temperatures already. But now you have a OK function to calculate what kWh to order through Piggy!
Now you can enable your limit again, if you need one. If you always go for 100% or set a limit, you can adjust your 8,9% to 10% and be sure that you planned for enough kWh, but not way too much, making Piggy schedule your session “more optimal”. Though it’s more fun to try and order only what your calculations say you need, and then tweak for each temperature 
Next time you want to charge and Tesla reports 37% battery you can make a logic card that calculates:
(Updated picture to be more precise)
And use this as input to your Piggy charging card.
You can use a variable for the “temperature percent” if you’d like…
When summer arrives, you can decide to redo your test to get a “more accurate” coefficient, and also redo your temperature percentages.
That’s one way to do it. You can also rely on the “remaining charge” inside Piggy for doing your calculations or visit a supercharger that tells you how much kWh was given to the car.
