> - I would like to have only the last element of the range filled.

I always thought that this would not be possible with reasonable
effort. Your question made me think again and it is possible, now I
can change my own use cases with sub-total :-) . See docstring and
take the procedure with vlen etc. from the TBLFM of the new ERT
test-org-table/sub-total here:
http://orgmode.org/w/org-mode.git?p=org-mode.git;a=blob;f=testing/lisp/test-org-table.el

Or read the docstring there and just take this copy that I used to
build the ERT:
|-------+-------+-------|
| Item  |  Item |  Sub- |
| name  | value | total |
|-------+-------+-------|
| a1    |   4.1 |       |
| a2    |   8.2 |       |
| a3    |       |  12.3 |
|-------+-------+-------|
| b1    |  16.0 |  16.0 |
|-------+-------+-------|
| c1    |    32 |       |
| c2    |    64 |  96.0 |
|-------+-------+-------|
| Total | 124.3 |       |
|-------+-------+-------|
#+TBLFM: @>$2 = vsum(@II..@>>) :: $3 = if(vlen(@-I$2..@0$2) ==
vlen(@-I$2..@+I$2), vsum(@-I$2..@+I$2) +.0, string("")); EN f-1 ::
@>$3 = string("")