Hi Snehashis,
here is the link to the official helpsite:
Calculating and Printing Grand Totals - Smart Forms - SAP Library
There are a few options to do, such as, do you want it to calculate it before the position is printed or afterwards.
IF are trying to do subtotals at each page or something like that, I can tell you, that this only work fine, if you only got a fix lines at each position. That means, if each position got one line, like in the fly-model of SAP, then it works great, but if there are textes in, like in an offer or something it will be very hard to achive and there will always some cases, where it doesn't work that way you want.
Why is that so, because the smartforms-protect logic decides during the processing, if there is enough space to print at one page, if not, it flows to the next.
Regards
Florian