Notional gain accounting treatment

Yes, provided the value of aggregate services exceed Rs. 50,000/- in any year.

Yes, without accounting how the tax liability be ascertained.

Notional gain itself means gain which is not monetary in nature,

As per sec. 56(2)(X) of IT act, any gain be monetary or notional is taxable.

Accounting in books means it is recorded based on its fair value.

Now since you know its a notional gain its probable that economic benefit inflow is certain. It must be recognised. If not make a disclosure. Its rarely that I see an example valid in accounting where benefits substituted for income.