Batch Sparging: Water Volumes

[kfm]

Is there any drawback to collecting wort from batch sparging in two different quantities?

For example, if I need to collect 77.6 litres of wort for the boil, the batch sparge calculator states that two equal quantities (38.9 litres each) should be collected: in this case mash volume would be 25 litres of liquor, topped up with 25.9 litres to collect the first lot of wort and then sparged with 38.9 litres for the second lot. If the mash tun can handle it, is there any reason why you cannot top up with 34.9 litres for the first lot of wort and 29.9 litres for the second lot?

[trucker5774]

Someone may shoot me down on this with the science behind it, but I can't see why it would matter. I sometimes do a 3rd batch due to a small mash tun or just dilute after the boil. If it works for me in those circumstances, I guess your suggestion should be fine.

[flything]

The main concept we’re going to be working with is that for the best
efficiency, the runoff volumes from your mash and batch sparge should be equal.

http://hbd.org/cascade/dennybrew/

[kfm]

Thanks to all, what you wrote John was exactly why I posted to ask and reading Flything's link and Chris's response I guess confirms that there is a science behind it but its flexible enough to work within. On the example figures I wrote, it would mean I can squeeze out 63 litres to the FVs using an old Burco for the last sparge water volume of 29.9 litres and avoid using an underback for collecting wort. It's all theoretical at present, been designed on paper and starting to get cobbled together in various stages. Without the advice on the forum there's no way I'd be building my own - cheers again