I have never taken to magnifying devices on stands & arms, or as spectacles, I found them fiddly and you have to realise that as the model is magnified, so is the paintbrush, scalpel etc, so the comparative size remains the same
But as I've got older (now 66) I realised something was needed. This is what I've done -
Went to the opticians and had an eye test; asked him to pay attention to my close up vision
Bought some close up/reading glasses with large area lenses; not a special order, just the largest they had in stock; got some lens cleaner and keep them free from smears etc
Made up two blocks of wood with some slight cloth padding on top, to rest my wrists, rather like a sign writers stick (forgotten its proper name)
Bought a few better fine paintbrushes; some of them have fatter rubber grips, I think I'll get that type again in future
I use a hand held magnifying glass to sort of size up or assess the work to be done - where demarcation lines are, and so on. Then straight away I do the painting, while it's still in my mind. Sometimes I secure the model with a bit of Plasticine or Blue Tack to stop it skidding around. Get the paint mixed and everything ready before you start
A friend who makes very good models, and is a bit older than myself has treated himself to one of those large illuminated magnifying glasses on a stand, and he reckons it's excellent