There are are other things that effect the hollow grind. For instance the angle at which the tool is presented to the wheel. Unless you are free hand sharpening. If a 25 degree cutting angle is wanted ( secondary bevel) then the primary bevel needs to be less than 25 degrees. A 20 degree primary bevel will allow more sharpening's than a 23 degree primary bevel, before the tool needs to be returned to the grinder. And the same holds true for a 30 degree angle. In all honesty, it doesn't matter what the primary angle is as long is, as long as it is less than the secondary angle is. Lets use 30 degrees. if the cutting edge is honed at 30 degrees and and the length of the shiny part is 1/16 inch long then the length of the side opposite the angle is 1/32 of an inch. And it doesn't matter if the hypotonus is 1/8 or 1/4 the strength of the cutting edge at 1/16 is the same as the first 1/16 of an angle at 1/4, because the they are the same.
My dove tail chisels have a primary angle of 15-17 degrees. My secondary or cutting angle is either 25 for soft woods or 30 degrees for hard woods. it doesn't take much effort to change the angle. The steeper primary angle allows the chisel to cut deeper into the wood with out allowing as much force to build up causing the chisel back into the marking gauge line.
However if one is using a chisel to chop out the Dato in 4x4 post then it has been somewhat cut out with a circular saw, then a cheap hardware store Chisel that came with a flat ground, would my first choice. . Application, application application.
In other words, if you have a 6 inch grinder, use it. You won't really gain anything by switching diameter sizes. Worrying about the circumference of an arc on a cutting tool is not of high priority when building anything.