I just found a chart that says 2 oz. of 65% cal hypo will raise FC by 1 ppm in 10,000 gallons of water. Assuming this is correct (someone please advise if not), then your situation requires

2 oz. * (25/10) * 20 = 100 oz. = 6 1/4 lbs.

so your 3 + 3 is probably good (although 6 at once might have been better).

The lingering question I have, however - help from a moderator here, please - is it valid to translate 16 fluid ounces = 1 lb. when you're talking about powdered cal hypo? I'm presuming that most people measure using a plastic cup marked in fluid ounces, not with a scale.