Yeah, that math is impossible, one can not know for how much the vouchers, in the end, will be traded, where the SPS price will or whether the DEC price will stay as high as it is. The game prints A LOT of DEC every day.
On top of all of that, the Airdrop is dependent on the totally staked SPS. More SPS staked, less Airdrop per staked SPS. Prisoners Dilemma.