However I would approach the beast from a different angle....

However I would approach the beast from a different angle. Question I need to find answer is at what FCF yield I would be happy for my investment for net 5 to 10 years, especially rising interest rates and bond yield environment.
I think it would be minimum to expect average FCF yield of 6% during next 5 years. And with the EV of 8B that would translate to 450-500m a year.

Well, one way to answer your own question, if you don't want to do it by the making of explicit forecasts, is to do it inductively, i.e., to reverse engineer the problem by solving for the required rate of growth in FCF, from the current level, to reach your desired $450m-$500mpa level (midpoint of $475m).

On a pro forma basis, the Free Cash Flow of the merged company is currently between $200m and $230m.

Starting with even the upper end of that range, to get to your $475m mid-point in five years' time would required a compound annual growth rate in FCF of around 16%pa.

Which is certainly no mean feat.

For context, REH's FCF over the past decade and a half (the period for which I have captured Reece's financials) as follows:

CAGR in FCF over the specified number of years ending in FY2017:
15 years: 13.3%pa
14 years: 14.9%pa
13 years: 14.8%pa
12 years: 12.2%pa
11 years: 10.3%pa
10 years: 24.8%pa
9 years: 14.1%pa
8 years: 12.3%pa
7 years: -1.9%pa
6 years: 11.9%pa
5 years: 8.5%pa