Many photographers do use dilute 1:3 sloppily as 1 part concentrate + 3 part water, one even may see such sloppy usage in some Kodak documents(posted 8376 days ago)But when it comes to serious instruction, Kodak has being very precise, for example on Tmax developer bottle " Add one part concentrate to 4 part water ", they don't say dilute 1:4
Ilford instructon is also precise, they say " 1 part concentrate + x part water "
The problem with the usage with slopy photographic ussage of "dilute 1:3" as "dilute 1 part to 3 part water" is that it is in conflict with the usage common in chemistry and medicine where a dilution of 1:3 means one third strength. Before photography was born, chemist had already mixing solutions for many many centuries.
Dilution 1:3 indicate a degree of thiness. (See New Oxford Dictionary of English )
Agfa also say " add one part concenrate to 15 part of water"
So IMo, dilute 1+3(water ) is better than dilute 1:3
The most strict difintion must be the one followed by chemists and pharmacists: dilute 1 part (of A )with solvent to make 3 parts
Because 1 part in volume of A plus 1 part of same volume of solvent does not always result in twice the volume of original, some times it may be more, some times it may be less.
A good example is 100 ml of ethanol plus 100 ml of water yields much less then 200 ml of dilute, if you want really 1:2 strength, more water is needed, otherwise, the dilition is more than half strength.
Therefore, in chemistry and medicime, the quatity of solvent is not important, sometimes you don't even know what it is, and don't need to know. The important thing is to control the final volume of the dilution.
In old days, photographic formular was quite strick, they told you to add such and such chemicals in 600 cc of hot water, disolve completely, then ADD WATER TO MAKE 1000 CC. Why not prepare 1000 cc of hot water outright ? Because, the resulting developer will be more then 1000 cc.
Fortunately, in photography dilution mostly refers to acqueos solution, and 1+ 3 = 4 parts.