Made a cooking mistake, how do I adjust the proportions? Urgent!?

You added 1/4 cup of extra water, so you added 1/3 more than the recipe called for.

So for the original comparable quantities you would need an extra 1/4 cup rice flour, but since you shorted that I would add 1/3 cup rice flour.

If 1/4 cup sugar is 4T, then add a little more than another tablespoon (1/3 of the original amount)

I'll make your life easier for you. Just throw away everything, go to the nearest trader joe's and buy a pack of mochi ice cream which goes for under $4 a box.

The thing about mochi is that you need to pound it for many many hours to make it soft and chewy. Otherwise, you are left with this brittle grainy nastiness....

I kind of doubt this is a cooking question, because I've been cooking for 50 years and have yet to see a recipe that calls for fifths of anything. I own six measuring cups and two sets of measuring scoops and none of them measure in fifths. There are 16 tablespoons in a cup. Divide 16 by 5 and you get 3.2. That is 1/5 of a cup. Multiply 3.2 by 4 and you get 12.8. 12.8 tablespoons is 4/5 of a cup. 12 tablespoons is 3/4 cup. If you put in 1/3 more water than was called for, then put in 1/3 more rice flour. Same for the sugar. If this is really a cooking question. I think it's maths homework.

Rice flour- add AT MOST 2/5 cups (I would put 1/5 to be safe. You can always add more, but you can't take any out!)

Water- add none

Sugar- put 2/4 (Equal to 1/2) cups instead of 1/4.

If it doesn't look right, scratch it. If you can't, just keep adding what seems to be missing until it looks good. Trial and error is all you can do!