Prince, the PayPal Website Payments Standard Integration Guide. http://www.paypal.com/IntegrationCenter/ic_documentation.html gives a pdf download.
The original modification to add PayPal to Q.C 1.4 passes several variables to PayPal from the templates/messages.tpl file. The relevant ones are - currency_code, amount, shipping. These variables are named by PayPal. PayPal looks at your member profile and works out the cost based on that. Importantly if there is no profile setting for some variables the default is used.
When the taxAndDiscount plugin 2.2 is used in Q.C you can set the rate of tax, tax to be calculated on the cost of the item or the cost of the item plus shipping (delivery cost) in the plugin configuration. At the moment it looks better to do this in Q.C rather than in PayPal.
Looking at the variables in taxAndDiscount I think the tax is in $aList[fTax] so If you want to pass the tax from Q.C to PayPal you could try the PayPal variable name with the Q.C variable added to templates/messages.tpl -