If you'd like exact rates, you can use belts of various speeds & smart splitters, eg.:
Need 300, so take a higher belt (eg one of the 455) and smart split off into a mk3 belt (270), and overflow the rest. You should now have a full mk3 270 belt and 185 remaining (overflowed). From that you can smart split into a mk1 (60), and split it in two (30, 30). Take one of those and merge it with the 270 from before and you've now got 300, with 125+30 remaining for more siphoning off. Basically, 300=270+60/2.
Similarly, you could do 2000=1200+780+60/3. The rest is left as an exercise to the reader.
Starsector is amazing, even though buying it on the website made me wonder if the storefront was made in the 90s.