It was fixed but it is no longer fixed since they have changed it few months back, now a user can get bigger amount than before (when it was fixed) especially if the user wager a lot or lose a lot.
The formula is now similar to monthly or weekly bonus if you ask me, but of course it is not 100% the same formula but similar because it is completely different than when it was fixed.
Get bigger amount? I think post or pre monthly having max amount depends on the VIP level ?
I don't know about this one but on my side the formula is still the same as before $15 for Platinum 1 and based on statistics my PnL is -$600 if what you say is true maybe it only applies to some criteria for example PnL -$10000 or more. Did you have a different bonus than usual in your last post monthly? if the difference is only $2-$5 I think it is still worthy to be called a fixed bonus.
I think we have discussed about it before, and AHOY had shown that he received something like $75+ for pre/post monthly bonus IIRC.
Other example is coming from myself, I used to get max $35 only no matter how much I lose and wager but there were few time when I received more than $50 after the change.
But as usual, every formula for bonuses at Stake will always remain a mystery and we will never know the exact formula how they count bonuses.