Brackish salt levels are not generally exact unless you have some particularly difficult species, or some that have been accilmatised. Go for 1.005 - 1.010. Yeah you need a hydrometer, but you can get one for about 7 or 8 pounds.
I have the SeaTest one on here and it seems quite good. Ignore the refractometer, they are generally only useful for fieldwork, should you neeed to do that sort of thing