I would try 2-3oz product to 12-13oz diluted water and see how you like it. I've used the Diablo Wheel Gel before and IIRC I used it at 10:1ish dilution ratio.
there are 128 oz. in a gallon .....128/16= 8 I mostly use 32oz. bottles that are marked off. Did you try to search on the forum? I think i saw one that explained it for 32 oz. maybe even 16 oz. and you can try to different dillutions of the same product like I do with my apc 10:1 and 4:1 you can try this Enviro-One Dilution Calculator I saw it on another thread