select b.aID
     , b.bID
     , b.Cnt
     , b.cntRec
     , b.grCnt
     / max(b.cntRec) over(partition by b.nID)
     * b.cntRec as dtSumma
  from (select a.aID
             , a.bID
             , a.Cnt
             , a.nID
             , max(a.Cnt) over(partition by a.nID) as grCnt
             , row_number() over(partition by a.nID order by a.aID) as cntRec
          from (select t.aID
                     , t.bID
                     , t.Cnt
                     , min(t.bID) over(order by t.aID desc) as nID
                     , lag(t.bID, 1, null) over(partition by case
                                                              when t.bID is null then 0
                                                              else 1
                                                             end order by aID) as lID
                  from @t as t) as a) as b

 order by b.aID;