UK Speed is largely rubbish although I'm told that there is some very good stuff if you have the right connections. If its any good you should be able to 50-100mgs easily, I'd start on the low side. Allot of what is sold as speed these days is either some nasty RC or cut so much you need a g or more to get anything from it. If its a good contact then that's fine at least your friend knows who they are.
For your use you'd be best just eating the stuff, it will last longer and be generally smoother.
As far as it being half the price of Ritalin this may or may not work out for you, the Ritalin is going to be much more consistent in strength than street speed. You really can't judge the dosing as you won't know how much actual active substance you are taking. This is kind of OK for recreational use but you are clearly using this for other reasons.
You don't have to answer but how come you need stimulants to get up in the morning? sounds like something you should be talking to your doctor about in the long term it's doubtful stimulants are going to be the answer, but like I said you don;t have to reveal your medical history to all and sundry.
Best Wishes