I used to keep my cache at 0 since my computer constantly has an internet connection. A recent server issue caused the Asteroids@home project to not issue new tasks correctly, and after it was fixed I changed my cache to 0.25 and max additional to 0.25.

What setting do you use, or what setting would you recommend? (my computer processes most tasks in less than 4 hours, so I'm not worried about overloading)

How many projects have you added?
Are they all allowed to fetch work?
All the same resource share?
Do you have one or more GPUs?
A beefy computer?

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I currently have 5 projects, but I'm only crunching on 1 at the moment. (1 of the projects has a lot of credit, so I'm crunching other projects to get them to all lined up. I'm just weird that way)

So, currently only 1 is allowed to fetch work. Once they all line up on the stat tab, I'll let them do their thing.

They all have the same resource share at 100%, so each project gets 20%.

I have 1 GPU, but it's built-in to my CPU, and only one of my projects actually uses it.

I'd say my computer is pretty beefy:

Intel i7-4770K 3.5 GHZ (Turbo 3.9, but that's disabled to prevent overheat)
8GB of RAM
BOINC is allowed to use 20GB of hard disk space.

In that case you could either set RS to zero on all projects (although I admit, not all projects have updated their server software to allow this yet), or continue as you do.

Is there a best setting if you allow one project at a time to fetch work?
Why not allow BOINC to make that decision and allow all projects?

My i5-2500K with AMD HD7870 runs every now and then, at a zero and zero cache setting, Seti and Einstein only, and only on the GPU. Every now and then is maybe in the weekends, if it's not too hot and if I am not playing TESO. ;)

Having just spent money on 4 more months of TESO, I guess BOINC'll have to wait some more.

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Yeah, I should probably just let the scheduler do its job. It just drives me crazy seeing 1 project's credit skyrocket because it runs GPU tasks while all the other CPU tasks stay behind. :P