"542 Mya the insolation would've been 95% of the present day. Bigger than any solar variation we've seen in recent times. Explains why the CO2 was so much higher yet the climate was relatively clement. "

So, 6 C higher average global temperature for millions of years is relatively clement?

I would guess you mean the the Sun's output of energy 542 Mya wouldn't be 95% of the present Sun's output.
And present output is:
"The answer from our measurements is 4 × 10^26 watts"
http://www.astronomynotes.com/starsun/s3.htm
The Sun's energy output (3.86e33 ergs/second or 386 billion billion megawatts).
http://www.nineplanets.org/sol.html
"The rate at which the sun emits energy (its luminosity) is around 3.8 x 10^26 Watts."
http://curious.astro.cornell.edu/question.php?number=389

Btw, as it happens, that 95% of 4 is 3.8.
Has there been an effort to get a very precise measurement of the total energy the sun produces?

Assuming today's output of the Sun is 3.8 × 10^26 watts
And 542 Mya it was about 3.6 × 10^26 watts. Would it be due it's size or it's surface temperature. Or some other factor? I suppose having a huge gas gaint orbiting a sun might have some magnetic affect

wiki [Solar constant]:
"It is measured by satellite to be roughly 1366 watts per square meter (W/m²), though this fluctuates by about 6.9% during a year (from 1412 W/m² in early January to 1321 W/m² in early July) due to the earth's varying distance from the Sun.."

"For comparison a 2% dip in insolation is sufficient to cause runaway glaciation - a Snowball Earth - according to the climate models used by Michael Hart's evolution models of Earth's atmosphere. His models didn't factor in CO2 build-up when the oceans froze over, so his Snowball events were permanent, but the basic finding remains the same. We're on a knife's edge - if it weren't for the natural CO2 cycles."

I think it's more complicated than you are suggesting, but during the recent few million years I think it's somewhat reasonable to say we are on knife edge in regard to entering ice age conditions- the normal of this recent period is having glaciers stacked a mile high on temperate land regions. If the Sun were warmer now, and this was a major factor, we should not be in such a cool period. And unlike CO2 there does seem a "runaway affect" due to increasing snow areas reflecting a significant amount solar radiation.

I believe that a major reason we are in such cooler period has do with the location of Antarctic continent. 70% of area of earth is ocean- if the poles did not have land masses near or directly on them- then global temperature would be significant warmer [somewhere around 5 C warmer].