luckyz. To begin with, please go to your "profile" and add your location so that we all know where you are. Growing tropical plants is so climate-dependent. Whoops, I see you are in Sacramento. Still add it to your profile.
I am no plumeria expert but have perhaps 4-5 dozen, all potted. So, what I tell you is what I have found works for me here in Mississippi. I start all my cuttings in a greenhouse with 70% shade. After a couple of weeks I will gradually move them out into the sun. They get several hours of early morning or late afternoon sun and I add an hour or so for additional sun every 5-7 days.
In my opinion your 1:1 mix is super-draining with all that perlite you have added. I think additionally, Miracle Gro already has perlite in it. Sooooooo, with all the perlite plus our hot weather, your soil is drying out way too fast. I think you need to modify your ratio and use 1/3 Miracle Gro, 1/3 Black Kow, and 1/3 perlite. That's not quite the formula I use, and I customize all my potting and garden mixes, but it is pretty close. Again, getting plants from a big box store, knowing that these plants have probably not had a smidgen of direct sun for weeks, I would give my newly potted plants shade between noon and 4:00. And like the cuttings, just add a hour every 5-7 days to acclimate the plants and leaves to full sun.
I don't know about the rock. I don't see anything wrong with using it. Again, this is just me, but plumeria do better with bottom-heat and not top-heat, so though it might not do any harm in having the rock there, it probably doesn't help. Mulching an inch or two would probably be better, though I don't bother doing that. The rock allows moisture to escape rapidly and mulch will help retain some moisture in that potting media.
We all seem to have our favorite way when dealing with plumeria and there are far more knowledgeable plumeria growers than I. I am sure you'll get some other ideas on what to do.
