Junko, you sound like a good soul ... and I'm sure everyone here wishes you all the best in trying to make some positive changes in your life. But I must chime in -- I don't think taking chronic low-doses of meth 'for weight loss' is a very good idea.
Even at low amounts taken orally, your body will build up tolerance. 'Finishing your bag' is not a strategy. Your physiology will adapt and after a period of time it just won't work as well (or at all) anymore. You might find yourself in the position where you will want to increase your oral dose -- just to get you back to where you were at the beginning.
And that's how the Express Train to Hell leaves the station.
I'm not even going to talk about the toxicity of meth. The havoc that meth does on your brain / axions / neurotransmitters is well-documented. Even low doses are going to be neurotoxic.
You also mention 'how much is 50mg? Two grains of rice?' ... argh. *shaking head* At the very least, if you're going to use an insane drug like meth, spend a few dollars / quid / kroner and buy a small scale. They're not expensive nowadays. A milligram scale in my area costs $20. Sure, it isn't lab-quality, but it's better than using 'grains of rice' as a measuring tool.
Instead of using meth 'for dieting', you would be much better off changing your lifestyle as several people have mentioned above. The best way to do this is to start small and make incremental changes. Keep a journal / diary of your habits and feelings, find out the cues for what is making you eat. Ask for help. Seek counseling. Go for a daily walk, eat healthy food and drink lots of water.
There's an interesting book I read recently called 'the Power of Habit', it talks about the reward system of the brain and our primal motivations for overeating / addictive behaviours, etc. ... I'm just recommending it as a place to start. There are lots of resources on how to make positive changes in your life.
FWIW, I'm not trying to sound preachy here. I've used speed. I have chronically used ritalin and adderall for no good reason. I have used meth orally before. Of all the shit I've tried, meth scared me. It's a drug that's too powerful, too awesome, and too addictive.
Take care of yourself.