What proportion of garden soil to coffee grounds should one have?

I think this will help you out, it tells you about half way down :)

https://groundtoground.org/2013/05/17/use-coffee-grounds-potting-soil/

I would think no more than 1/3rd grounds to 2/3rds compost soil...........

Just sprinkle the grounds lightly over the soil. Remember, coffee grounds are acid and you want to keep the correct pH of the soil. If you don't plants will grow, but not to their best. The 1/3 to 2/3 is way too much for even acid loving plants. Get a soil test done!! Contact your County Agricultural Extension office. They have soil test kits for under $10.00.

Used coffee grounds are actually used to straight to start seeds in. But to be safe keep it 5 parts soil to one part of coffee grounds.

http://homeguides.sfgate.com/use-coffee-grounds-vegetable-gardens-24003.html