Sell side valuation

Just trying to get my head around doing valuations in M&A deals.

Let's say for example, I am the IB on a sell-side of a transaction.

Parent company P is selling their subsidiary A to another company B. So we are advising the selling side to get the highest price on the disposal of A.

Now my question is. when carrying out the valuation for this.
Do you also need to take into account that if B purchases A, there will be synergy arising? (I would assume yes, because this synergy affect would be the selling point for raising the deal price)
Or does the sell side only consider the value of the company to the shareholders if it wasn't sold?

(i.e. if using a DCF, do you forecast future cashflows based on the firm being operational as it is, or take into affect additional revenue/decreased cost due to the transaction?)

Are you supposed to use a merger model from the sellside as well?
then how would you assume how the buying company will finance the deal? (i.e. stock/share/debt?)
are we supposed to take a guess at what the buy side's target capital structure will be for the deal and discount the cashflows from the buyers perpective and then come up with a sell price?

or just consider the selling company without thinking about the buy side when setting a sell price?

I'm not fully sure if I am getting my question across properly.
I guess in big picture, I'm wondering how much analysis of the buying company you have to do when deciding on the sell price for the disposing company.