But what is the lowest requirement for the balance to have the highest full base reward?
There isn't one. If the wagered to free ratio is high enough you could have a zero balance and still get full base reward
Obviously, I meant the case where one is not gambling
We brought in the $50 and $100 limits so that the system doesn't penalise people with old accounts that made a lot of free-rolls back when BTC was around $200. It is very difficult for them to wager enough at $3,500 BTC to outweigh their total free winnings. So yes you are correct as long as you keep that $50 balance then your base reward will never be reduced
So how exactly is this $50 threshold determined?
You see, the balance is shown in bitcoins, so no fiat is involved in the process, right? It looks like you have to take this balance and calculate its value in fiat. What exchange rates are you using and where can we check them? And while we are at it, it would be good to have an option or switch that would allow to see the balance in dollar terms - again, to see where one is standing with this threshold in mind