Estimate the corpus you need to retire and the monthly SIP to get there, allowing for inflation.
Expense at retirement E = monthly expense × 12 × (1 + g)^years. Corpus = E × [1 − ((1 + g)/(1 + r))^N] ÷ (r − g) × (1 + r), or E × N when r = g. The SIP closes the gap after existing savings grow.
If the post-retirement return equals inflation, the corpus is simply E × N.