10 str 1 luck machette 20 attacks outside of vats, 10 in.
You will notice not only the blindingly obvious damage bonus for vats, but that low str benefitted much more than the high STR, trippling damage, while high STR doubled. So it's most likely a constant value being added.
For the hell of it:
10 str 1 luck with heavy handed perk added in 18 attacks without VATS, 9 in VATS.
And no, none of these scenarios were red shield damage.