I was just thinking the same, Nan
Teresa, I've been looking all around today and can find nothing that recommends a 7.5mg dose of mirtazapine for any size cat at any frequency. Everything that I see acknowledges a 3.75mg dose (1/4 of 15mg), but also says that 1.88mg (1/8 of 15mg) is still effective but with fewer side effects. This particular article gives the frequency of 1.88mg dosing as every 24 hours for young, healthy cats, and every 48 hours for older/liver/kidney cats. Especially with what happened last night, I really urge you to press your vet for cyproheptadine, or compounded 1.88mg mirtazapine capsules (since it is almost impossible to split a pill into eighths) or transdermal mirtazapine. Personally if I were you, I would lean towards crypo instead.
Here's the article that I recommend you read:
https://www.americanveterinarian.com/news/wvc-2017-mirtazapine-for-appetite-stimulation-in-cats
But here is the summary:
"In conclusion, mirtazapine works as an appetite stimulant in cats. Current dosing recommendations for mirtazapine in young, healthy cats is 1.88 mg/day. In older cats and those with chronic kidney or liver disease, 1.88 mg can be given every 48 hours. This dose, while effective, results in fewer undesirable side effects than the traditional 3.75-mg dose."