If speed is the primary issue, practice with a metronome to get about 300 cycles per minute. Marcel Mule taught 16ths at quarter=72 (see his Leduc edition of Ferling studies), then later decided 76 was a better speed (Rousseau bio). Prof. Rousseau's is, by his own description, closer to 80. Of course, speed must be artistically varied along with all other parameters, but this gives you a good ballpark. Given the 300-ish speed, you should practice triplets at 100, 16ths at 76, quintuplets at 60, and sextuplets at 50. This variation will give you some flexibility and make certain that you're not only conceiving this in 16ths. Ultimately, it will not be metronomic at all and, as other posts have rightly suggested, will be used with artistic discretion and reflect your study of vocal and string vibrato as well.