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.