According to the docs the syntax of the command is like so:

mods.enderio.SagMill.addRecipe(IItemStack[] output, float[] chances, IIngredient input, @Optional String bonusType, @Optional int energyCost, @Optional float[] xp);

mods.enderio.SagMill.addRecipe([<minecraft:planks>], [100], <minecraft:log>);

So the following should add minecraft:wheat -> vanillafoodpantry:flour_portion with a 20% chance for minecraft:wheat_seeds.

SagMill.addRecipe([<vanillafoodpantry:flour_portion>, <minecraft:wheat_seeds>], [100, 20], <minecraft:wheat>);

However in-game the 20% chance for the minecraft:wheat_seeds is actually outputting 100% of the time.

The floats should be decimals, 20 is treated as 2000% in this case. Use 0.2 for 20%

https://crafttweaker.readthedocs.io/en/latest/#Mods/Ender_Tweaker/SAG_Mill/

The docs here should show that then, rather than using integers which is misleading.

Alright, thanks.

I did not make those. The only official documentation for EnderTweaker is the one here on Github