Frequencies confuse me

Can anyone help me with a simple easy answer help me understand frequencies. for example.

i read that 

HP = High pass LP= Lowpass

4 inch speakers should be crossed from 100 hz - 250hz  HP

5 and a quarter inch 70-180hz HP

6.5 inch 50-180 HP

6 by 9 unch 40-120hz HP

tweeters over 3k hz 2nd order HP

Subs 35- 100hz LP

are these figures accurate? and another quesiton when a speaker says lets say 6.5 inchers that its frequency response is 50 hz -20k hertz does that mean you should at least cross it at double the minimum like in this example 100hz or so? also if anyone can tell me about components that come with their own crossovers like lets say 6.5 inchers that have a response of 48hz -25Khz on the amp you put them on HP right? and what would be the frequency you set on the amp or does the crossover it came with do that and also should you add a passive crossover after for the tweeters or does the crossover it came with do the job properly?

My thoughts about this are that there are no exact numbers. When tuning your system, certain settings will sound better. What you have is a good estimate on what speakers can do and where to start setting crossovers, but there are many variables to this, like power, enclosure/mounting loacation and other speakers in the system.

Generally one would still set the HP filter on the amp when using a passive crossover, hopefully you will have the ability to tune this filter, on what is a called variable crossover. For example my 6.5" components have a certain frequency response like 38 hz up to 25Khz . I have my amp tuned down to about 65 hz on the HP filter and they sound good. I could play these speakers down to 38 hz, but I am sure it wouldn't sound too great. So again, the best thing in the end is to go by your ears and how the speakers react.