ThumbsUp = (TotalEarnings - (TotalWins * BaseReward)) / BaseReward
= (27 - (2 * 9)) / 9
= (27 - 18) / 9
= 9 / 9
= 1 thumbs up
The base reward (neutral or thumbs down) for Gold decks is 9 electrum. You get double that for a thumbs up.
Just take the base reward, and multiply it by the total number of wins, then subtract that value from your total earnings. Divide that by the base reward again, and you get the number of thumbs up your deck received.
We can use the same formula for your Plague deck, which won 8 games and received 90 electrum.
ThumbsUp = (90 - (8 * 9)) / 9)
= (90 - 72) / 9
= 18 / 9
= 2 thumbs up
This formula won't work if your deck reaches the top 10, because earnings for top 10 decks get multiplied by another amount depending on which position it's in at the time. This is why you'll sometimes see these threads give crazy (or even negative) values for the thumbs up/down calculations.