This Excel Spreadsheet analyses the SolisManagerExecutionHistory.csv file which contains Solcast data
It calculates the ratio between Forecast and Actual Solar data for each half hour.
One method compares the averages of the above, while the other compares the maximums of the Forecast and the actual solar data.
It turns out that using the maximum values gives more stable values because using the maximum values corresponds to full sunshine conditions, so the effects of weather are minmised.
Derek Johnson