Make the terms back to back where the risk has to pass down

If the employer can charge you for a delay, your subcontract must let you charge the subcontractor who caused it; if a variation is recoverable upstream, it must be recoverable downstream too. Line the payment, programme, variation and damages provisions up against your main contract, so a risk cannot fall into the gap and strand itself on you. Back to back does not mean identical, it means no orphaned risk.