So I don't brew with rice but I do make rice wine (which is a beer) but has a very different mashing process. In traditional rice wine making, you rinse the rice until the rinsate is clear, soak for an hour (I usually invert the first two steps) and then steam until the rice rather than boiling so that the rice doesn't stick together so much, but you'll still have to declump it manually after cooling. Stop steaming when there is no white core when you cut open the rice kernel.
Alternatively, you could boil in so much water that you end up with rice soup, maybe 4 parts water to 1 part rice. I've done that with oatmeal to fully gelatinize the oatmeal for higher mash efficiency. I would probably go with this approach. For estimating the amount of water you are adding to the mash, you could soak rice overnight, rinse/drain and then add a measured amount of water.
What Scott Janish describes appears to be the normal way of cooking rice and then adding to mash.
I guess I wouldn't adjust mash volumes unless you are adding rice with liquid water to the mash or unless your rice is >15% of the grist.
A note about rice: Long grain rice wlll ferment out pretty completely. Glutinous rice, e.g., sushi rice, will provide body.