Reallocation of headquarter after acquisition // How to value tax benefit?

Hi guys I am currently doing a DCF on the Allergan Pfizer deal. The US-based Pfizer was about to acquire the Ireland-based Allergan in order to reallocate their HQ to Ireland and to benefit from a significant lower tax rate.

But how can I value the tax benefit which will occur after Pfizer is about to lower their tax rate from 35% to almost 15%? Just calculation a DCF on the benefit or how would you do it?

Thank you in advance!

Best