Custom rounding

If you want to combine your code, you can useTable.AddColumn once and apply the rounding logic to both columns within the same transformation :

RoundedVolumes = Table.AddColumn(Grouped_Table, "Rounded_Volumes", each
[
OEM_Volumes_Rounded = if Number.Mod([OEM_Volumes], 1) >= Rounding_Factor
then Number.RoundDown([OEM_Volumes]) + 1
else Number.RoundDown([OEM_Volumes]),
DISTRIBUTOR_Volumes_Rounded = if Number.Mod([DISTRIBUTOR_Volumes], 1) >= Rounding_Factor
then Number.RoundDown([DISTRIBUTOR_Volumes]) + 1
else Number.RoundDown([DISTRIBUTOR_Volumes])
],
type record
)

If you want to expand this record into separate columns, you can use Table.ExpandRecordColumn:

ExpandedRoundedVolumes = Table.ExpandRecordColumn(RoundedVolumes, "Rounded_Volumes", {"OEM_Volumes_Rounded", "DISTRIBUTOR_Volumes_Rounded"})

If you want to improve the performance, you can work with lists instead of the entire table  because it avoids you loading the entire table into memory for each operation. 

oemVolumes = Table.Column(Grouped_Table, "OEM_Volumes")
distributorVolumes = Table.Column(Grouped_Table, "DISTRIBUTOR_Volumes")

roundValue = (value) => 
    if Number.Mod(value, 1) >= Rounding_Factor 
    then Number.RoundDown(value) + 1 
    else Number.RoundDown(value)

roundedOEMVolumes = List.Transform(oemVolumes, roundValue)
roundedDistributorVolumes = List.Transform(distributorVolumes, roundValue)

RoundedVolumesTable = Table.FromColumns(
    Table.ToColumns(Grouped_Table) & {roundedOEMVolumes, roundedDistributorVolumes},
    Table.ColumnNames(Grouped_Table) & {"OEM_Volumes_Rounded", "DISTRIBUTOR_Volumes_Rounded"}
)