Ratio of Subwoofer wattage to main amp...

Is there a "best" ratio of main amp power to subwoofer amp power or does that matter? For example, if my main amp runs 100WPC, what should be the minimum that my subwoofer amp should be? It seems obvious if my subwoofer is very powerful that merely rolling down the power or volume is all that is needed, but what about the other way around?
There's no blanket power ratio per se, as there are many variables to consider, and it would depend.
First, the amount of power needed to properly drive your subwoofer might greatly depend on the sensitivity of your subwoofer itself! If your main speakers are less sensitive than that of your sub, you may need less power to drive the "more sensitive" subwoofer to a "proper balance" between the amp for your speakers vs. sub in a given scenario. Conversely, if your sub is a "less sensitive" design than say your main speakers, you'll need more power to your mains proportionately. So it can depend, even on just that consideration.
Not even considering other factors, there's simply SO MANY DIFFERENT SPEAKER SENSITIVITIES AND DESIGNS, that you couldn't give a "best ratio" of power between them for your purposes. It's entirely likely you could be using a "low sensitivity", "low efficiency" sealed box, single woofer design for your sub, and high sensitivity horn speakers for your mains! YOu might, say, need only 20 watts to "blow your ears out with the horns, but need 1000 watts for your sub to keep up! Or, you may have something like a highly sensitive ported, multi drive subwoofer and some 85 db audio monitors your using, and would maybe need 200+ watts for those, and like 100 watts for your sub for your room and listening levels! Too many variables to consider
Avantgarde speakers truly represent the best bi-amplified speaker combo with active woofers. It's recommended to drive them with 10...20W/ch mid to highs amp and the sub amp is 150W/side.
So far I haven't heard any bi-amplified system that can outperform them.