Beth, this is a complete shot in the dark, but if it is a systemic fungal or bacterial issue, there are two approaches you might consider:

One is a copper treatment of some sort - Phyton 27 is the most common product used, as it is - to a degree - a systemic fungicide/bactericide. I have no first-hand experience with it, as I avoid it due to the fact that some plants (some dendrobiums and most thin-leaved types) don't go for it, and to some, it's out and out toxic.

The other is a biological control. The colonies of bacteria and yeasts in products like Inocucor Garden Solution can actually colonize the plant as well as the medium, protecting it from pathogens and actually eliciting some self-protective responses.

SLC is not the type that is known to be prone to necrotic spots due to Ca deficiency.
It's certain bifoliate cattleya species I believe.
Plus, usually you will see it on the young leaf that is just emerging not the one that's fully extended.

Calcium helps strengthen cells and makes them less vulnerable to attack by fungus and bacteria. I add the eggshells when I first get the orchid, usually when potting it into my LECA or red lava rock. I had crushed oyster shell but it kept slipping through. Indeed, it does take time to break down and is not an immediate source but, once it begins the process of releasing calcium, it does a very good job. I add new every few months so there is no interruption.
If this is bacterial, there is a product on the market that is used on pear trees for 'Fire Blight' that might help. If it is a fungus, one of the best products is Captan (the brand we use on our fruit trees has copper in it).
You can also treat everything but the roots with Isopropyl alcohol to disinfect it. I have successfully saved orchids by using it. If there are other orchids nearby, they should be treated, too, to prevent the fungus and bacteria from being reintroduced.

I treated my sick Lc. Mari's Songs with generic Cleary's systemic fungicide.
I'm not certain what it was, other than it was fast moving and I believe that it was due to being exposed to cold temps during shipping. The leaves had black sunken spots that grew over a week. Then I tried repotting them; a couple days later, most of the roots rotted. I believe my case is/was different than yours though! Here's a pic of the top of a leave. The time span was a couple days in between.

Btw: physan didn't work for me. But the gen. Cleary's did. I'm not sure what's wrong with your plant, but I hope you'll figure it out :)

Cleary's is good stuff, too. I have never used it but I have heard great things about it. I am glad your Lc recovered. :)