Turnover in the reference period is generally considered to be the turnover from June to September 2019 and this turnover will be shown on your VAT return.
If you submit your VAT return on a quarterly basis, then the months of June to September fall into two quarters. In this case, take the turnover for the second quarter (April to June inclusive) 2019 and divide this amount by 3 and add this to the turnover for the third quarter (July to September) 2019.
If you were exempt from VAT in 2019, you have to calculate the turnover from June to September 2019 and back this up with annual accounts or other evidence from the accounting of the 2019 turnover data.
If your company was established between 1 April and 15 November 2019, you will use the turnover figures for the 4 months following the month in which the activities were started.
If you started your business between 15 November 2019 and 1 March 2020, you will calculate the turnover for the period after the day you started your business until 15 March 2020, you will divide this amount by the number of months whose turnover is taken into account and then multiply it by 4.