That's exactly the right way to find the total 'Payload' weight that the hull can can carry when floating to the LWL; Load Water Line. AKA Plimsoll Line, nowt to do with footwear (cos someone extracted the 'P'😮) 😉
Subtract from that the total weight of all the gear to be fitted inside (especially batteries!), all superstructure and rigging, deck fittings, crew figures and anything else you are gluing on.
What's left is the amount of ballast you need to reach the LWL.
I always try to make sure as much as possible is used for useful payload, such as larger batteries to extend the endurance👍
25lb is A LOT!😮
Plenty of scope for big batteries and special functions etc 😊
Cheers, Doug 😎