Hi Matt,
I wouldn't worry just yet. I don't keep Spur-thighs, but 285 g. for a 3 year old doesn't seem bad to me. It sounds like you are measuring her properly, but do remember that the Jackson ratio was devised to determine whether a tortoise is of sufficient weight to go into hibernation, and it isn't necessarily an indication of whether the tortoise is the right weight. It sounds like you are giving her a good diet, so well done.
We usually aim for an average weight gain of between 1g - 4g per month -- and that is just an average (some months they will gain more and some less), so the fact that she gained 1g this last month shouldn't be cause for worry. Do you have a good reptile vet near you where you could take her for a check-up (it really does need to be a vet with expertise in reptiles, rather than an ordinary vet, and if you don't know of one we have a list of UK vets on our website here:
http://www.thetortoisetable.org.uk/tort ... dEp09FryM8 (and if you can't find one near you, let us know and we'll try to help find one). The vet can also do a worm test for you if you bring a fresh sample of poo with you (you can leave some overnight in a sealed container in the fridge if necessary).
A light load of worms is nothing at all to worry about, and in fact most vets now don't recommend treating a light load, but if she has a heavy load then it should be treated. When tortoises have a heavy load of worms, they often lose their appetite and are lethargic, so it doesn't sound like she is suffering from a heavy load, but you can never tell and it is wise to test if you have suspicions. Protozoa are another matter, and the vet can test for those too.
Let us know how you get on, and please don't hesitate to contact us if you have any more questions or we can be of help in any other way.
Nina