SQL Server 2014 - ISNULL/IS NOT NULL

Hi Experts,

This there a better way to write:

cast(sum (case when (nn1.iplong is null and nn1.plugin is null) and (sa.plugin is not null and sa.iplong is not null) then 1 else 0 end ) as Decimal(18,2)) / cast(count(sa.scandate) as Decimal(18,2))* 100 as [Percent 30],

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I have several CASE When statements that are similar.

something like:

Cast(Sum(ISNULL(nn1.iplong  and nn1.plugin) and IS NOT NULL(sa.plugin and sa.iplong ) then 1 else 0 end ) as Decimal(18,2)) / cast(count(sa.scandate) as Decimal(18,2))* 100 as [Percent 30],

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[/code]

Is this correct?  Seems this would make better performance if several of these Case when statements could be replaced?

Thank You for helping me...