If your calculation is based on a threshold create a Metric for NI Threshold and budget the monthly amount for that

Create another Metric for NI over Threshold and use a Formula to calculate Wages - NI Threshold

Store the NI rate in another Metric

The NI expense account is then a formula that calculates NI over Threshold x NI Rate

The NI liability account is NI Expense x -1

In Cashflow Settings, Cashflow Forecast, Advanced set the cashflow type for NI expense to None and NI Liability to Schedule and Monthly (or whichever frequency makes sense)

Your cashflow forecast will now show just the liability movement and your P&L reports will show the expense