How do I calculate the value of a firm after a acquisition? :)

If you think the merger should result in a better working company, then buy more shares.
If you think otherwise, sell.
If you don't know, investigate. Call your dealer and solicit info from other dealers.
Look back on the financial stability of the two firms as far back as what a reasonable valuation might be done. If the two have made profits according to their best years in business, then you could assume the merged company should do as well together.

Oftentimes, when mergers happen, the stock price may dip because they will usually see where they overrun each other's business. This may cause reduced employee numbers.
This should not be taken as a cause to be concnerned. The businesses may have more employees than will be necessary for the merged firm.

When a company announces how it is saving a lot of money because of belt tightening, it often causes the stock price to go up. Its what the ensueing quarters (3 months-6 months) result in its financial position.