That i decide based on my percentage of large mid, small cap. To clculate that u can enter all ur SIPs and additional purchase in value research online my portfolio then go to analysis page-u will know the allocation. i try to maintain 60-70% in large,20-30% in mid, <10% in small.. As of now my large cap stays around 56%.So next additional purchase i will do with large cap based fund(in which already SIP is running). So i rotate(If necessary) and buy.. I dont sell for small rallies at all.. I accumulate units when market down.. When market raise i juz skip SIPs and try to take back the amount i spent for additional purchase................... Again if market goes then i may consider selling(minimally).. So it may remain for many years..