RE: How Much Your ROI Has To Be To Return Your Invested SBD When Buying Upvotes(Based On The Price Of SBD)?

You can get it more easy I think...
It's obvious that for every 1 SBD you do as an investment you have to receive
(c x 100[%]) / (0.5 x 0.75 x (c + 1/c))
as a return in STU
where c is the value of 1 SBD in USD.
Right now 1 SBD is 2.17 USD. Thus you would need 220% return in STU for every 1 SBD you invest. It means the payout has to be more than 2.2$ to receive more than your invested 1 SBD (worth 2.17 USD).
☺️ @peekbit