Presumptive tax section and refund of excess amount paid

Hello , My advance tax payment was Rs. 12 lacs in the financial year 2017-18. However, as my turnover is less than 2 crores and my tax liability would be less if I opt for the Presumptive tax section (sec 44AD) . I have been advised to take benefit of this section. . My tax liability under presumptive scheme is 4 Lacs and consequently my claim for refund would be Rs. 8 lacs . I wish to ask the following a) How much time does the IT Dept. take normally to process refunds of larger amounts? b) In the present circumstance where the income is more than 8%, lets say 20%, , is it permissible to declare the presumptive percentage of 8% and meet my tax obligation by paying tax on such 8% , or is obligatory to disclose the actual percentage. c) If I declare the net profit to be 8% , will the IT Dept automatically accept what I am declaring while processing the refund d) can I be asked to prove that my income is not greater than 8%