opting of presumptive income

No its completely OPTIONAL...

Suppose You have gross income of 75 L but loss of 10 L, then how can you declare 8% profit? Thats not the purpose of the section. 

It is to help self assesment people who have good income declare a fixed percentage and forget about full compliance like audit etc. IT dept will not bother u... 

But even if you are below 44AD/44ADA if turnover/ gross receipts is within limit(ie 2cr/50lakhs) but have less than 6/8% profit you can do an audit and file under normal ITR rules 

opting 44ad is optional , but once if you have opted then you will have to show a minimum 8% profit for a periode of 5 consecutive assessment years...