How to calculate an intangible value of a software product

But then the accurate intangible value of the software product cannot be calculated.

Accounting principles are simple. They are developed to measure everything accurately. And there is no measurement of cost beyond that. What ever way is convinient, people follow relevant principles. Example, depreciation. SLM method is slower than diminishing balance method. Yet companies choose that because their tax planning is not in hurry to claim all depreciation immediately. Similarly, if you include your salry, you will get a tax benefit through amortisation. There is no intagible value of an asset, ACCOUNTANTS ASSIGN A VALUE TO THEM. 

For further queries on taxes, use the tax section here. Many accountants know about taxes for startups.

Hello, I want to learn a little about API integration, where can I read about it because I myself can not find anything

Although it is very important eg. Toad application in Oracle integrated with databases, you don't have to learn anything not unless there is a good job demand. 

@ kokker There are many forms of API integration. Depending upon what you are integrating with you can normally get the steps from the platform with which you are integrating.