```ts
export function doseTotals(log: [string, string][]): Record<string, string> {
  const totals: Record<string, number> = {};

  for (const [reagent, amount] of log) {
    if (!/^-?\d+(\.\d{1,3})?$/.test(amount)) {
      throw new Error(`Invalid amount format: ${amount}`);
    }

    const parsedAmount = parseFloat(amount);
    totals[reagent] = (totals[reagent] || 0) + parsedAmount;
  }

  return Object.fromEntries(
    Object.entries(totals).map(([reagent, total]) => {
      const roundedTotal = Math.round(total * 1000) / 1000;
      return [reagent, roundedTotal.toString()];
    })
  );
}
```