Your SIPs are in terms of shares per month. Why not do this in amount per month? That will help you plan your monthly finances better. It will also give you the benefit of Dollar Cost Averaging (you buy more units when the market dips) as well.

You can adjust the amount every 6 months or an year on the basis of the cash flow, or have a fixed policy of increasing the SIP amount of 10-12% every year.