Let's say you have 10 labs all with 10 of each science pack in them. You begin researching a tech that requires 75 science. Eventually the labs will come to a point where they've all used up 7 packs, a total of 70. How do we hit the remaining 5? In my observations, what'll happen is that the labs all begin researching at once, but eventually they'll stop when the progress bar hits 100%, probably around the time they're halfway done this current round of science.
As best I can tell, the progress bar for research is filled by depleting science packs like fuel until it hits 100%, but the individual labs are just a means to provide fuel for that gauge, their own progress bars do not necessarily mean anything. I make this analogy because I've observed research complete when I have a single operational lab that is not yet done its progress nor are the science packs in it depleted. So if the lab reaching 100% is not what ticks up the progress bar, and a science pack being depleted is not what ticks up the progress bar, then the progress bar is being influenced by something that cannot be directly observed.
Back to the 10 labs, now we have 10 labs all with science packs that are 50% depleted. If we pick up those science packs, they "combine" like ammo magazines, so we have 5 full science packs now instead of 10 half-full ones. That would make splitting them up again considerably harder. But, does it matter? Because if the research progress bar is just a fuel gauge and labs are funnels, will the progress bar fill the same if I left those half-full science packs in the labs versus powering only half of them with full packs now? If so, then what is the purpose of the progress bar for the lab? If I build a web of labs where inserters move science packs between them, I notice the labs momentarily begin working when science packs are put in them and before they're removed. Does this mean that by the time the science pack ends up at the end of the line, that final lab receives a science pack that is not quite full while all the labs before it ticked up a tiny bit? If yes, then how do productivity modules fit into this? Will the final lab be unable to complete a research cycle entirely and thus productivity modules in it may not fill the productivity bar?
I leave these questions unanswered because I don't know. When I try to micromanage my labs due to having few science packs, I inevitably notice that over the course of a playthrough that my labs all end up containing science packs in differing stages of depletion, and I don't understand why or how, and if it affects the efficiency of my research or not. Here's a combined screencap of the science packs in three different labs on my current playthrough.
And an anecdote on how this confusion and/or inefficiency can affect gameplay. On another scenario, I had a limited number of blue science packs, so I put them towards laser turrets and calculated that I had just enough to finish the research. I did not - the research progress bar ended up at 99%, the lab's progress bar was at 99%, and I was out of blue science packs. Somewhere along the chain, a tiny tick of progress was lost and I was left without laser turrets before the scenario progressed.
Again, I don't know for sure how science works, I'm not a programmer or coder and haven't looked at the technical aspects of how Factorio handles research. What I do know is what I've observed with my own eyes - I have only one lab actively researching, and research completes when the lab is not yet done its progress cycle and none of the the science packs in it are depleted. As I said before, it seems like labs are funnels used to pour science pack fuel into the progress bar, and the amount left in each science pack or the progress of each individual lab is irrelevant against filling the progress bar. If that is the case, then the true method of researching new technologies, and the impact of limited-duration science packs, labs, and productivity modules, on that process should be made clearer to the player so we can better optimize and micromanage our labs.
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