The gold standard for feeding your guinea pig is Critical Care. Unfortunately when disaster strikes, you may not have any on hand. Below is my "recipe" for homemade syringable food.
First, I take a mix of fresh vegetables, carrots, kale, celery, strawberry, etc and blend to a pulp in a smoothie blender.
I then mix that 50/50 with Timothy hay pellets, blending and adding water as necessary to make it syringable.
The final product is about 1 cup blended vegetables, one cup pellets, two cups water.
I dehydrated some of the mix and and found that each 10ml serving contains ~ 10.1g water and 1.7g solids. I read online that minimum nutritional requirement Guinea pigs is 100ml water and 20gm solids per day (CC recommends 27g) per kg. This equates to about 120cc per day of slurry per day, per KG.
Hope this helps!