The truth is, that most fellows follow a Mikawa progression including a Koma with a tomonagura as their final slurry source. Nearly everyone has found that the sheer quality of a super tomo is finer than that of the grit of even the best Koma Mikawa nagura and this is why their last slurry is from a tomonagura.
The Mikawa nagura can provide a progression of grit that you can build a method upon, and thousands of fellows do this and they have since 1952, the year that Iwasaki-san of Sanjo visited the Mikawa mine for the first time. These same fellows into the modern era were led to this method by several different guru who promoted super hard Honyama base stones and a Mikawa progression as the only way to hone razors, and that most of these guru were, for their own reasons, strongly set against the use of a diamond plate to raise a slurry from the base stone for honing.
The first proponent guru was Iwasaki-san of Sanjo, a famous blacksmith that forged superb blades in the 1950 thru the 1970s, an era before the invention of the diamond plates we now all use. During his lifetime the famous Honyama mines of Kyoto had mostly closed, and the stones used by the finest carpenters in the world, those there in Japan, and the multitude of barbers still honing their razors with these domestic natural stones cherished their investment in their stone collection beyond our current conception. I would agree that these early craftsmen would have cringed at the idea of using a diamond plate to raise a slurry from their precious commodity if they did possess a diamond plate. Most of these mostly men could only afford to buy a few stones in their lifetime, stones were rare then and the market was dictated by a couple of handfuls of wholesalers in and around Kyoto. These craftsmen lapped their stones with the 3 stone method, and tradition told them to leave their tools sharp before they went home at night. This was a different breed, and I worked with a few of them as a carpenter apprentice in the late 1970s. We worked 6 days a week from 8am until generally 5pm, or 13 days in a row if the job at hand required it, 12 months a year. These men ate, drink and breathed their tools and their trade. Iwasaki-san was of this generation, he never saw a diamond plate and although unequaled in his trade, he was very traditional until 1952.
I use this date of 1952 as a marker. This is the year that Iwasaki-san actually visited the Mikawa mine site for the first time, met with Asano-san the head of the Barbers Union and a wholesaler of barbering tools and supplies to discuss and observe the different qualities of the white (shiro) stone firsthand at the mine. Before 1952 the main use for shiro Mikawa nagura was in sword polishing, and that was mainly only Koma and Buchikou. Barbers and carpenters only used small softer pieces of Honyama stone to raise a slurry on their base stone. Iwasaki-san, a kamisori blacksmith with a scientific bent was looking for a way to codify the honing of his razors, and he had examined and realized that there were different grades of Mikawa nagura that might be useful to study further.
I test hone and shave from each and every stone I sell, and I do this by testing the actual grit of the base stone by raising a slurry made with a well worn out #600 Atoma diamond plate that acts similar to a #1200 Atoma, but mellower. This way I am testing the components of the base stone itself instead of from an alternate grit source like the Mikawa nagura. The Diamond Nagura DN method is the purest way I found to unlock the hidden qualities of the Honyama base stones, and this is one reason why I own lots and lots of Mikawa nagrua but do not promote it so strongly.
Superior quality Mikawa nagura like Koma is just about the same cost in dollars as a super quality piece of Honyama nagura or a larger Honyama base stone per gram, and Koma is near impossible to buy now. I have found that a slurry made using a DN will perform each and every step that a full 4 stone Mikawa progression will, and faster for about the same cost. This DN slurry one stone progression can be performed with the slurry made during the lapping of your Honyama base stone. Many fellow laps their base stones with a diamond plate and just wash the slurry down the sink, unused as waste. Yet they will take a $50 piece of Mikawa nagura to make a slurry and use it for just a few minutes, followed by two or three successive nagura slurries and wash those down the sink as well. Using the already present Honyama slurry created from lapping provides your with the best of both worlds, a flat stone and a highly grit rich slurry component. Creating a slurry made with a Mikawa nagura provides you with slightly dished or irregular stone surface and a short term single use (1 of 4 in a progression) slurry component that is rated on a scale of 1 to 4.