Second all of these. The official US dilution rate is (in US measurements) 1 fluid ounce per 5 gallons; this is 29.6 ml to 18.9L. Rounding it down gives 1.57ml per litre or for you, 7.85ml per 5 litres (round it up to 8ml). Don't worry about the froth; it is supposed to froth up so it can effectively cling to and sanitise all the surfaces. Just tip it out (or save and re-use: you can do that) and the residual froth will not hurt your wort. If it really bothers you, do it the day before and let it sit overnight; the froth will subside (but only do the fermenter lid on loosely: you don't want a vacuum to collapse the fermenter as it cools down).
Best of luck with it!