The "Highest Active Percentage" that you see in Resource Monitor is simply the highest active time seen BY Resource Monitor over the time it gathers the statistics, and isn't something that the system keeps for you. What you can do is gather the relevant statistic yourself on a periodic basis and use it to work it out.
You can get disk usage statistcs using WMI; an example PowerShell script to get the current PercentDiskTime statistic for all disks at this point in time would be:
$wmi = Get-WMIObject -Class "Win32_PerfFormattedData_PerfDisk_PhysicalDisk" -Filter 'Name = "_Total"'
$wmi.PercentDiskTime
You can filter on the expression to get the statistics you want, or change the -Filter 'Name = "_Total"' parameter to change to the disk you're interested in, or even remove it altogether if you want stats for everything.
You can use this to work out the highest value your script has seen so far.