I haven't noticed any adverse effects with the temperature fluctuations you described. In fact with hova-bators, it seems almost impossible to avoid them. At those temps, however, you're going to hatch out a lot of boys. If you want more girls, aim for the low 80s.
Your lay box plan sounds fine. Just be aware that some leos will eat vermiculite. If vermiculite starts appearing in the stools, switch to something like bed-a-beast or sphagnum peat moss (the dirt like kind). I make my lay boxes out of cool-whip comtainers. I just cut a hole in the side about half way up (height depends on which size container I'm using) and fill it just about up to the hole with bed-a-beast.
Albey's incubation instructions with the perlite worked great for me last year. I think that my hatch rate was a lot higher following those instructions and using perlite than it had been when I incubated on vermiculite.
Good luck and I hope this helps,
Alice