I know it's different in the UK, but, when Amber was started on ritalin, they started on a low dose to check her reactions to it. When I started 20 mg twice a day. I went right on that dose as the doctor trusted the judgement of my wife and myself. I met a person at a continuing ed. conference who was on 30 mg three times a day. And he was a psychiatric professional, plus he was also a bit on the "chubby" at around 6 feet tall.
They should start you off with around 10 mg, twice a day to see how you respond or maybe 5 mg. Then they can always bump the dosage up as time goes on to see how you tolerate it.
For the record, sometimes I do not take the afternoon dose, depending upon what I'm doing that afternoon. Then if I'm having a long day of travel or whatever, I'll take a third dose, but, no later than 5-6pm. All this with my doctor's approval.