How much do I throw away as foreshots on a 25 L wash run?

Work from the charge, not from a flat percentage of the collection. The rule is 2.5 mL of foreshots per litre of wash charge, with a minimum of 25 mL.

For 25 L of an 8% wash: 25 × 2.5 = about 63 mL discarded, and rounding up costs almost nothing.

Two notes on the reasoning. The foreshots discard exists because acetaldehyde and ethyl acetate concentrate at the very front of the run, and those compounds have well-documented unpleasant acute effects. It is not about methanol, which a clean sugar wash barely contains. And the discard is a safety margin, not a precision cut: nothing bad happens if you throw away 150 mL instead of 63, while the reverse is a headache in both senses.