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the game basically says to not convert with 1 resource

No, the game says it's 1 runik = 2.67 wheat. Then it says there is a 70% tax. So it should charge 4.539 wheat after tax but instead it tries to charge 10.6 wheat for 1 runik...

That's a 400% tax.

The math probably tries to multiply something by the number of non selected resources would be my guess. Either way, it's wrong and should be acknowledged and fixed by either correcting the tax rate shown or correcting the formula if you think the current numbers are in a good spot.

base conversion (without tax)
2.67 wheat = 1 RNK

actual conversion (1 resource with tax)
10.6 wheat = 1 RNK

actual conversion if without tax (1 resource)
10.6 wheat = 4 RNK (Tax this by 75%, you get 1 RNK)

Tax rate is working perfect as how the developer wanted it. The game basically says don't convert with 1 resource only. Make progress and unlock all resources to get as low as 5% tax.