As mjm says, you can thin it out later, so add whatever liquid you think you’ll need.

I’m in a half Japanese family but live in London. Most economic way for me to get this kind of curry roux is in a massive 1kg (2 pound) block. I just slice away what I think I will need at a time.
I don’t ever make up sauce the way you’re intending to in advance because it keeps rather longer unhydrated.
Also worth knowing that you don’t actually have to cook it for 5 minutes - you can easily shred it on a cutting board, add it to any stock or liquid as you wish, stir and it will be done in about a minute. The way I use it is to add it to my already cooked/boiled ingredients, but right at the end of the process as a final flourish.
Once shredded you can actually use it on already cooked rice along with chopped onions and anything else you want in there (like spinach or mushrooms) to make a “dry curry”. A spoon or two of liquid will help the shredded curry roux bond with the rice.
I find it a very versatile cooking ingredient, and yes, shredded and melted into liquid will quickly turn it into a sauce for chips, bratwurst, omurice or anything else you want.