If your going to use the coefficient method you shouldn't use 1/20th of the
ACH50 as the base infiltration. It is supposed to be an average value for
typical exposure etc. You could use the BLAST eqn but you will need to
adjust the inputs. I'd suggest looking at the pressure drop across the
house at the peak wind condition and see what the Pa difference is across
the windward and leeward walls. Assume a linear drop with pressure from
your ACH50 value and pull out the "basic infiltration" from that.

I agree that 1ACH at 50Pa is challenging with standard practice construction.
The average new home in NZ without anything special is about
6ACH at 50Pa.That said, two weeks ago, I witnessed a blower door test of
a passive house
(1st time builder) that was less than 0.3ACH at 50Pa the first time we turned
on the blower door. This was the first time this builder had attempted an
"airtight" house - after fixing a few oversights it was at
0.26ACH at 50Pa.The structure was designed to be sealed properly and he
used ProClima's
tapes and membranes. If you build a SIP house it is also likely to be
under 1ACH50 without trying too hard.

Jason Quinn