In 2017 HPE spun out MFGP. I never sold this stock but recieved a 1099B for the full amount. What should the cost basis be (the sale amount on the 1099B) ?

The distribution of the SeattleSpinco shares, (that was the company actually spun out by HPE), to you was a non-taxable event.  22.22% of your basis in HPE should have been allocated to SeattleSpinco.

The SeattleSpinco/Micro Focus merger IS considered a sale of your SeattleSpinco stock and the "Proceeds" of the sale is the FMV of the Micro Focus ADR's you received.  But there's a wrinkle here: while you must recognize gain on lots where you have a gain you cannot recognize a loss on lots where you actually do have a loss.  Netting of gains and losses is not permitted.  So you'll have to make your calculations on a lot-by-lot basis.  If you have a gain, you simply subtract that lot's basis.  If you have a loss you set your associated basis equal to the "proceeds" so that you report $0 gain/loss.

This is all sorta explained in the Micro Focus Form 8937 here
https://investors.microfocus.com/media/1438/micro-focus-form-8937-10317.pdf
and you should read it.  That will also help you understand what your basis in the resulting lots of Micro Focus will be.

The Form 8937 suggests that the appropriate "fair market value" to use for the "proceeds" calculation is $29.34.

@TomYoung Thank you SO MUCH for your complete answer.  I appreciate your help greatly!

This is way too late for you but for anyone else looking at HP cost basis, including if you participated in the HP stock purchase plan back to 1959!, the HP alumni association has a spreadsheet that will do ALL the math on the coast basis for you. You will need to know when you accumulated your stock which either ComputerShare (the old agent) or Equitini (post 2000 employees) should know.