Looks like you got a series 3 mini! They are pretty simplistic, I have a basic guide for you that I wrote myself:
All tamas grow up based on care, never luck. The mini has a baby stage and an adult stage. The baby stage is always the same, a little blob that bounces around. But here are the adults:
From left to right you have Mametchi (perfect care), Kuchipatchi (good care), Masktchi (bad care), and Nyorotchi (horrible care). The last one is Oyajitchi, which all the characters will turn into after a week of living. The only exception is Bill (the ?), who's the secret character. You can get Bill from giving a Mametchi perfect care.
The lifespan of the mini seems to be 2 weeks in my experience.
You can turn off sound with A+C. Press B to view the clock. If you need to change the time, press A+C. Then use A to scroll through hours first, B to confirm and then A again for minutes and B again to confirm. If you don't confirm a change time, and leave your mini on the changing time clock screen (you'll know because the numbers will be blinking), then your tama is technically paused. It will not grow, get older, get hungry, etc.
I've noticed that sometimes pressing C a few times (not on clock screen) will make your tama dizzy, which is kinda cute.
Since there is no status bar, pressing A only gives you the option to feed your tama a meal and snack. You don't have to watch your tama devour these everytime btw, you can just keep clicking B to continuosly feed your tama until they shake their head. When they do this, you know that they are full. This is the only way of knowing whether or not you've feed them enough so don't be scared to keep feeding them. Them shaking their head does NOT decrease care or make them unhappy.
A acheives just about everything including feeding meals and snacks to curing illness to cleaning poop.
To answer your question in short, there's no scale, you just feed both options until it refuses to satisfy it's hungry and hapiness.
There are more complex tamas being re-released (kinda) in stores soon, check the thread under "Comments and Rumors"; they're the P1s and P2s from the '90s.
Hope I helped!
edit: here's the link to the thread!
https://www.tamatalk.com/IB/topic/195309-2017-tamagotchi-p1-p2-appear-in-gamestop-inventory-system/