I know it used to be 35 or 36% (for tyranny) but that was based on bushel price of $32. Now that bushel price is more like $39, what is the optimal tax rate (it should go up).
As you mentioned it varies with bushel price. You're good to stick with 35% as a general rule. I don't feel like digging out my optimal tax rate table to look up at which point to adjust your tax rate.
The optimal tax rate not including bushel price is 34%. Although increasing it to 35, or even 36 will only lower your pop by about 1.5% and 3% respectively, forget how much it would increase your income by though.