I don't think that there is one correct answer to your question, which is a good question. All that can be said is the different experiences that different members have had over the years with sinemet, so i will tell you my personal atory, for whatever it's worth.
During my first 3 years i had the "honeymoon period", where 3 25/100 regular release did fine to keep me medicated well enough all day long. But I also took 200mg of Amantadine /day and 10mg of selegiline/day too.
From about year four to year six, I had problems with on/off syndrome and those were the years that i tried all the agonists, Permax. Requip and Mirapex. i stopped selegline no problem but carried on with the Amantadine. I had some "not too bad " results with Permax and Mirapex, but Reqip was definitely not for me. Comtan "threw me in a loop" it made me terribly dyskinetic. Eventually ,i figured agonists were "just not for me".
Then we tried to mess with sinemet intake. At one point in year 6 to 7, I went up to 1800 mg total sinemet normal release /day. I found this to be a bit overwhelming and so, lowered my sinemet intake, month by month, down to 800-900mg of total l-dopa/ day, along with 200mg of amantadine.
Well, here I am, Just finishing year 10 and take 2, 50/200 controlled release/day, and from 4 to 6 25/100 per day of normal release/day, and 200mg of Amantadine. I have now "stabilized" on this dose which means 800 to 1000mg of l-dopa per day. This has kept me "happy" , for about a year.
That's all i can tell you except that up to 2 grams of L-dopa/day is, or can be, tolerated well by the human body, before dyskinesia limits its usefullnes. That's from reading PD literature.
Interestingly there was a report of two people taking up to 25 grams!!! of dopamine/ day, just to get "high" from it. It didn't kill them but they had wild dyskinesias. Hope this enlightens you a little bit. cs