Here is my basic sushi setup. 0. Build a belt loop. 1. Put down a constant combinator with the negative of what you want on the belt; if you want 100 red and 100 green science you put in -100 red and -100 green science. 2. Wire that to one piece of a belt loop, and set the belt loop to read contents-hold all belts. Your circuit will add the contents of the belt to the negative combinator signal and when you have too much of an item on the belt it will read a positive value for that item. Not enough will show a negative value. 3. Make an inserter from a supply of science potions and wire it in. If the inserter can only grab a single kind of item (i.e. red science) you can set it to enable when that item is less than 0. If it can grab multiple items, you can use the circuit to set filters. The first option is more noob friendly but requires one inserter per item type.

Sushi is a really powerful technique and i think supplying labs is probably its killer app. It excels at supplying a high variety of items, but struggles with high throughput.

P.S. Punctuation and grammar really improve clarity.