work out an average daily drop. e.g. if you see 20 varroa after monitoring for 5 days, then the result should be 4 (=20/5).
If you did a drop count on multiple hives, then calculate the average for the apiary as follows. Add up the total number of dropped varroa for all the hives monitored to get a number, Sv. Multiple the number of hives (H) by the number of days (D), and work out Sv/(H*D)