The easiest thing to do, would be a buy a gallon. Use pool math to figure out what the expected rise in FC would be and give it a go. Like if half a gallon should give a 3 FC rise and you only get 2 then you can figure it's 2/3 what you estimated in strength.
That's what I did before I got the SWG dialed in. For me, at the time, it was better than 6%. But of course there is an error rate to consider so 5% was probably more realistic. Since getting the SWG worked out I haven't needed to supplement.