Thanks for the suggestion. I gave this a go but I'm not sure if it really achieving the purpose. When I try this it simply uses the largest mash volume possible, plus max sparge and then whatever is left becomes top up water. It does not seem to affect IBU calculation and it feels a bit 'hard wired'. What I was looking for was for the software to still advise the correct mash volume, sparge volume etc for the recipe, but understanding that my final batch size is larger than max values of my equipment and so to put the rest as top up water and calculate IBU accordingly. Using the hardwired method it was suggesting I use a 27L mash for about 4.5Kg of grain.
I did play around with this in quite a few ways, but I could not get a result where it seemed to be utilising the concept correctly as it does in Beersmith. I do appreciate you going to the trouble of providing screenshots etc, but with my fiddling around I haven't been able to get it to play ball.
My scenario if anyone is interested was just a basic recipe of 4kg trad malt, 200gm crystal and 200gm light munich - all joe white. I know from Beersmith that if I add 18gm Superpride @13.9%AA that I should get 22.3 IBU in a 29L batch using grainfather as equipment, set with 3L fermenter topup in profile. That is with 15L mash and 18.5L sparge suggested from bsmith. In this scenario the top up water would be 3L. I do this with a robobrew also so total top-up for both in the final 58L batch becomes 6L.