Get the total usage (in percent) between two specified dates
if I understand your requirment correctly your command should be like this, if not please provide example result.

Select DeviceName, (Count(ConnectStatistics)* 100 / (Select Count(*) From [dbo].[Statistics])) as Score, pcname
From [dbo].[Statistics]
where datetime = '2019-02-27 09:40:00'
Group By DeviceName,pcname