The maximum amount of vitamin C a thal should take per day is 250 mg. The recommended dose is between 100-250 mg. Vitamin C helps to release more iron into the blood where desferal can remove it. Too much C can release too much iron to the point where it can cause clumping in the heart and adversely affect heart function. No studies have been done on the affect of vitamin C with other chelators, so it is not known what effect, if any, there is.
I think orange juice is an excellent source of C and two glasses at the start of your desferal is a good idea, but you are right in saying that the 250 mg pill is more than you will get from orange juice. It might be best to take a lower dose pill along with the orange juice. A question I have is, does it make sense to take more C or orange juice or source of C while doing the desferal, in addition to what is taken at the start? Also, would it make sense to take a vitamin C pill that is time released? Is it more effective if the C is released over the whole period of time desferal is taken?