You can reuse. There should be instructions for addition of time for developer, but it isn't very much (as C-41 is quite sensitive to developer time changes), a few seconds per roll per liter.

One-shot is recommended for extreme repeatability but there's nothing wrong in reusing with time increase, it is an approved way to process. The deviations from the standards are low, probably lower than with most minilabs. Very well maintained prolabs are different and if you want to get the same level of quality, you should do one-shot and carefully control time, temperature, agitation, solution injection and drain times etc. In that case, the Unicolor blix kit is not recommended anyway, so just reuse, save money and be happy...

It is up to you whether you want to take the smallest possible volume (240 ml?) at a time and reuse it fewer times with greater time increases per roll, or mix the used developer in larger amount of "working solution"

I tend to use 500 ml as a working solution and reuse it up to four to five times, or even more if I don't need guaranteed results. The reason I use more than ~250 ml is that I want to have enough working solution for the cases when I want to develop more than one roll at a time.