Kalidurga, I know you hate math but this isn't too bad. Go to http://sheldonbrown.com/gears/ and enter your chain rings (48 and 34) and then enter each cog for your cassette (he may have it in the drop down list). Look at the results. If you put each gear combo in order highest to lowest, you'll see where you overlap. Some combos may be duplicates; others may just be a hair different. You'll suddenly see that it doesn't make sense to cross gear beyond a certain combo.

Also, to get lower/easier gears, you'll need a larger cassette (and a new derailleur and chain). But larger cassettes have larger jumps from one cog to the next. Some you'll have to decide which you want more.