70 sounds about right for 32mm tires. See what the sidewall says -- if it says MAX 80 -- you could try 80 since you're a bit heavier. I wouldn't exceed the MAX, however.

That being said...if it's a new tire freshly filled with air, it's going to be about 78% nitrogen and 21% oxygen, 1% other gasses...

The oxygen will get lost quicker than the Nitrogen. So you figure if you lose 10 PSI, you might be at 80% nitrogen, 19% oxygen, and further and further down the road, you will start losing less pressure because there will be more and more Nitrogen in the tire and less oxygen.

So for the first month or so, you will notice you lose more pressure.

PS I'm not a scientist, just base this off logical thinking, evaluation, trial and error, and opinion.