The phrase "assumption being" is correct and usable in written English.
You can use it to introduce an assumption that you are considering as part of your argument. For example, "Assumption being that the economy will improve, there is reason to believe that investments could yield a good return."
The assumption being, of course, that you would be one of the new people.
Even parliament continues to use 'he' in legislation, the assumption being that that will refer to everyone.
Most answered "no", the assumption being it was the shoelace, or the beggar's foot, or the car.
Double that when the invitation is for dinner out -- the assumption being that no restaurant past the city limits is worth it.
They were able to grow to sizes many times bigger than a country's GDP – the assumption being that the market would be self-regulating.
No one has been charged with his murder: the working assumption being that this was just another case of mistaken identity.
None of the players are compatible with Macs, the assumption being that there is no reason for them to try to compete on Apple's home court.
'Character' is a big buzzword in American politics, the assumption being that honesty, integrity, loyalty and decency in private will translate to public life.
Morning TV relies on constant repetition, the assumption being that most viewers, unlike the President, will be too busy to watch for long.
And following such exercises, utilities are likely to ratchet down plant security -- the assumption being that there will be similar advance warning before a real attack.
He tends to treat highly developed, highly reified, in a sense, doctrine as idolatry – the assumption being that you're always wrong about things.
