I'm not sure how can you get the $20 because you wager $600 only, if your calculation is real then I'll be very happy.
I'm platinum 3 in Stake and my average weekly wager amount is around $20k-$30k.
If I use your calculation, should I ask them to give me $500 for weekly bonus? LOL
Just FYI, my weekly bonus is around $45-$55 with $20-30k wagered.
I think you are expecting too much from the weekly bonus.
With your statement above, I believe, the OP is expecting too much from stake.
As stated, this bonus is given as a good faith, however, the OP experienced a deviation from his usual weekly bonus.
I think, he should contact the online support because they are fast in responding their players.
Or he can even post in the stake thread here. I don't think this site site will steal a small bonus to their players.