SELECT mtg.idTemaGestion, mtg.detTemaGestion,
count(mtg.idTemaGestion) as total
from mac_gestion mg
inner join mac360_mac_gestion mmg on mg.idMacGestion = mmg.idMacGestion_FK
inner join mac360 m on m.idMac360 = mmg.idMac360_FK
inner join mac_tipogestion tg on tg.idTipoGestion = mg.idTipoGestion_FK
inner join datos_nomina dn on dn.idDatosNomina = m.idDatosNominaRecibe_FK
inner join site si on si.idSite = dn.idSite_FK
inner join mac_temagestion mtg on mtg.idTemaGestion = mg.idTemaGestion_FK
where mg.error = 'si'
and si.idSIte in (2)
and tg.idTipoGestion in (1)
group by mtg.idTemaGestion
order by total desc;

create table Errores (
    Titulo varchar(100),
    Cantidad INT
)

insert into Errores (Titulo, Cantidad)
VALUES  ('Error A', 15),
        ('Error B', 85),
        ('Error C', 25),
        ('Error D', 64),
        ('Error E', 10)

SELECT *,
    C.ACUMULADO/Total
    FROM (SELECT   t.Titulo,
                   t.Cantidad,
                   @acum:= @acum+ t.Cantidad AS ACUMULADO,
                   tot.Total
                   FROM Errores t
                   JOIN (SELECT @acum:= 0) r
                   JOIN (SELECT SUM(Cantidad) as 'Total' FROM Errores) tot
                   ORDER BY t.Cantidad desc
          ) C
          WHERE C.ACUMULADO/Total <= 0.8


drop table Errores

╔═══╦═════════╦══════════╦═══════════╦═══════╦═══════════════════╗
║ # ║ Titulo  ║ Cantidad ║ ACUMULADO ║ Total ║ C.ACUMULADO/Total ║
╠═══╬═════════╬══════════╬═══════════╬═══════╬═══════════════════╣
║ 1 ║ Error B ║ 85       ║ 85        ║ 199   ║ 0,42713567839196  ║
╠═══╬═════════╬══════════╬═══════════╬═══════╬═══════════════════╣
║ 2 ║ Error D ║ 64       ║ 149       ║ 199   ║ 0,748743718592965 ║
╚═══╩═════════╩══════════╩═══════════╩═══════╩═══════════════════╝