Its still 750K - 1.5M per employee, its not like engineers are less valuable if they are bootstrapped. But a better question is given the businesses involved what values change. Does the acquiring company become more valuable? Is there less competition? Is there intellectual property involved? Customer contacts?

A much better way to start this process is to do a fund raising exercise. How much would you expect the company to be pre-money and post-money, and why? If you have been unsuccessful raising money it is possible your post money valuation expectation is too high. At the end of the day though its value not a 'rule of thumb' for these things.

People will use the 'rule of thumb' to cross check the value of their offer. If they feel like they are only in it for the employees and offering more than a 'typical' acquihire they will need to understand why that is. If less then that is another data point.