As the self-confessed FF newbie...I will still throw my $0.02 in:

I think that the tin has those instructions on it to cater for the "lowest common denominator" IYKWIM. The people with no common sense who would think "yeah that'll do" and mix it up willy nilly if they weren't very strictly told not to. I can imagine how upset little tummies would get if they were given a different blend of formula and water every single time and if it was not mixed with care. Let's face it, there are people out there who would do this and I believe this is part of the reason why the instructions are so strict.

I would have no problem with following MCHN/paediatrician instructions to mix the formula a bit differently if that was what they thought would be better for my bub for whatever reason. There has to be a valid reason why it is such a common suggestion. Constipation?

The formula tin would never suggest alternative ratios than the standard to prevent people self-diagnosing and monkeying about with the mix without professional advice. If anyone is having doubts I think they should go back to the source of the advice and question them further rather than stop doing what they have been advised to do.

Hope that makes sense.