The water split is controlled by the mash profile. So that is where you want to concentrate your efforts.
The water recommendations in the Anvil manual are geared towards people who are new to all-grain or all-in-one units and who are not using brewing software. There is a way to mimic these instructions, but really it is not needed.
I have the 6.5 gal Anvil and generated my own equipment profile. A step-by-step method can be found in the 'equipment' topic. When I mash, I set the mash profile for a BIAB profile, which means all the water is included in the mashing and there is no sparge.
To set up for a sparge and rebalance the mash water to be more favorable, you can take one of the single infusion mash profiles, duplicate it, and then modify it to work better for you. If you are doing a 'pour through the grain bed' sparge, I would recommend putting more of the water into the mash and minimizing the amount of water used for the sparge. In my testing, the pour through displaced the free wort already in the grains, but did not actually pull any significant amount of sugars out of the grain.
Anyway, take your copy of the mash profile you want to modify and open it up. You can then edit the mash step for the initial infusion to increase the amount of water by setting the 'water/grain ratio' value. Most sources recommend not being under 1.25 qts/lb and I have found better results being up around 1.75 qts/lb (I am assuming that you are using English (US) units -- if not, then you can convert or just let me know and I already have them in metric units). Where you want to set this is really up to you and feel free to play around with this value to get it right for your system and how you want to operate it.
If you want something which is closer to the Anvil recommendations, then I would choose a BIAB mash profile. Modify your equipment profile to add 1 gal of water as 'top off water for the kettle'. Now the program will hold back 1 gallon of water for you to sparge with and apply the rest of the water needed to the mash. When doing this, it would be good to set your grain absorption under 'options' > 'advanced' > 'biab grain absorption' to around 0.8 to 0.9 fl oz/oz of grain as a starting point. You will need to measure your water in versus your pre-boil volume to calculate exactly what you are getting, but this is a reasonable starting point which should not leave you short on volume if you allow the grains to drip back into the Anvil for a while.
I hope this is clear, but feel free to write back if it is not.
