Sekerob, your method to compute WU's needed will work fine for hours.
If you want to compute how many WU's you need for x days use:
WU needed = x days * 24 / 7

To calculate how big your WU cache should be to get that many WU's use:
cache in days = WU needed * 7 / 24 / number of cores

For example, lets say I need 15 more days to reach the next level on Rice.
WUs needed = 15 days * 24 /7 -> 51.4 which I round up to 52 work units.

If I have a quad core machine then I should set my cache to:
cache in days = 52 WU's needed * 7 / 24 / 4 cores -> 3.8 days

I've noticed when using Ubuntu and Boinc 6.4.5 with i7 processors it doesn't request enough work to match the cache size. So check that you get the needed number of WU's by counting them on the Boinc Manager Tasks tab.

Hope this helps.

Steve

To guard against overcommitting yourself with Rice-wu, even if you've been running only Rice so the DCF is "stable", don't use larger cache-setting than 9 days.

Ingleside,

[utterly off topic] If you've got a solution to the fixed hour run time projection, either server side or client side, probably both, that works ideally, but not probably with older clients too [when the server side could be adapted to], suggest to provide the code to the developers. Many will thank you for closing that gap in the cache 'set and forget' crunching realm. Till then, the wide range of CPU speeds contributing and daily mix varying, will make those fpops go up and down... just for a few more weeks, which we'll survive. If you have, a solution, bring it to the devs. [/ utterly off topic]

Hmm, I don't see anything wrong with the calculation...

To me, it seems reasonable that a quad-core that needs 15 days of cpu-time to reach a badge would need roughly a 4-day cache...