The 15 September instalment is due at 45% of estimated annual liability, which for a seasonal business is a guess dressed up as a calculation.
Under-paying costs less than you think
Interest under section 234C runs at 1% per month on the shortfall, for three months. That is often cheaper than blocking working capital on an estimate you already suspect is high.
The judgement call is whether your Q3 and Q4 are genuinely predictable. We work that out with you rather than defaulting to the safe number.