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.
Or the requirement for the highest tier of the free-rolls payout table which should mean the same thing as I see it (for those who don't gamble)? You say that $50 balance guarantees the full base reward. Does it mean that it is large enough and thus doesn't depend on the ratio of total free winnings against wagered amount and balance? That is, no matter how long you play free rolls, your full base reward won't be reduced to 33.5%. If it will, how much should be added to the balance to "make life full again"? And after all, could you write that formula so that we always knew where we stand in respect to this reward?
I'm not going to give the actual formula so as not to encourage people to try and game it. Sometimes we find a lot of people doing that and have to tweak the formula.
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.