actually to decide how much the tension should b use, 1st u mus know the weight n quality of ur racquet as well as ur own strenght. some racquet can hold up to 30 lbs where as some only manage to hold up to 27-28 lbs. it will b hard for u to return the shuttle if u don hv enough strength n ur tension is too high. i myself is using BG66, 24lbs n my racquet is Ace Power Titanium 68 Long.
(FYI, make sure u remove ur racquet string imeadiately if it has broken, bcoz the unbalance tension will bend ur racquet or make ur racquet goes out of shape especially when u put it in ur car boot n park ur car under the hot sun.)

n for where to hold the racquet, u can adjust urself while during the game. It is recomended to hold the upper part of the handle when u serve , push n drive the shuttle. bcoz in this way u can react faster with better power n the shuttle will move faster. however, it better to hold at the bottom if u want to lop or smash, bcoz in tis way ur racquet will hv greater swing n the force to push the shuttle also higher, so u'll achieve a more powerful smash n solid return.