As far as the auto enrolment rules are concerned, you must use the post sacrifice salary to calculate whether or not minimum contributions have been met.
This is best covered by an example:
Pre sacrifice salary is £50,000. Contributions are 3% employer and 5% employee (including tax relief). The employee wants to use salary sacrifice to make their £2,500 contribution. Their post sacrifice salary is therefore £47,500, meaning that the actual employer contribution of £4,000 (£1,500 + £2,500) represents 8.42% of the £47,500 salary, not 8%.
It therefore needs a bit of maths to calculate in advance what level of sacrifice would result in the minimum contribution being met when measured against the post sacrifice salary. Most providers require the minimum to be based on the pre-sacrifice salary, accepting that this results in the contribution then being more than the minimum required when measured against the post sacrifice salary.
In the above example, the employer is making the minimum contribution and the employee is using salary sacrifice to make the employee contribution. It's unlikely that providers and/or the Pensions Regulator will allow the total required contribution to be funded by employee salary sacrifice. The employee must always have the option to go for a conventional employer/employee contribution basis and there must be no coercion into using salary sacrifice.