I am no expert in this, as it is only my personal preferences, and I havent exploited the full potential of the smaller wave foilkites yet, thats why I was not precise.
I can tell you what I use and prefer, but I dont think this is gonna answer the question.
For a 12 or 15 m2 UL foilkite I only use 30 m lines, but this is because I only use these in 4-6 knots of wind.
For tubekites in less than 10 knots, I use 30 m lines also.
For an 8 m2 Peak4 just under 10 knots, I use 27 m lines.
For tubekites 3½ to 9 m2 (10 knots of wind and up) I use 26 m lines, have tried shorter but does not work for me.
For single skin foilkites 3,4,5 m2 Peak4s, I use 21 m which works just fine, but havent tried shorter, sorry.
So personally it seems I ride 3 to 5 meter shorter lines on foilkites, than tubes.
And now you might understand why I would not be "precise", as I have an idea it is just me
Although, as I have found this difference, there could also be a difference for others, regarding "how low can you go" in linelengths, foils versus tubes?
Just a lot shorter than mine of course
That was my thought at least, and why I did not put numbers out...