When I first started HRT I was advised to start on half patch (matrix type - not reservoir - shouldn't cut those), but when I asked the pharmacist he said manufacturers don't recommend it, so he couldn't endorse it. I did start on half a patch & didn't come to any harm & in common with other members have continued to reduce my dose gradually by cutting measured pieces off whenever I've needed to stop e.g. for surgery. That seems to minimise withdrawal effects compared with the one occasion when I had to stop suddenly.
I've just changed to a different type of regimen with a view to increasing dose & my consultant agreed without hesitation that I could start on half patch.
Just did a quick trawl online & there's mixed advice but there are medical sources who endorse it, albeit some with caveats, as shown in the following links & extracts . . .
https://www.nhstaysideadtc.scot.nhs.uk/TAPG%20html/pdf%20docs/Linked%20Documents/Tayside%20Menopause%20Guidelines.pdf"To achieve lower doses . . . women can . . . cut up matrix patches (not reservoir patches). These are all unlicensed uses of HRT. If this is tried with sequential preparations, irregular bleeding will occur."
https://clinicaltrials.gov/ct2/show/NCT02264743(Royal Brompton & Harefield NHS Foundation Trust sponsored trial in collaboration with Chelsea & Westminster)
"Group B administered ½ Evorel Conti patches - trans-dermal Estradiol 25 mcg/norethisterone acetate 85 mcg (½ Evorel Conti patches) daily for the duration of the treatment phase (24 weeks)."
https://harrowhealthcare.co.uk/stopping-hrt/"If you’re using patches, you can ask for a lower strength or simply cut the patch in half down the middle. If you wish to reduce your HRT even more slowly, taking a third or a quarter off your patch will work too. Pharmacists may tell you this makes the preparation unlicensed, but it works extremely well!"
Sorry, that doesn't answer your original question loonarider, as to how precise the cut should be, but it does at least show that it's recognised practice.