One of the features of Shipping by Zone/Class is the ability to insert formulas into each type. Below are some quick examples for you to copy and adapt.
Store Manager > Settings > Shipping
Charge £3.50 for the initial item and £3.50 for any addition item added
3.50 * [qty]
Charge £4.50 for the initial item and £2.00 for each additional item added
4.50 + ( 2 * [qty] )
Charge £8.00 for the initial item and plus 10% of the order total of which it is at least £4.00
20 + [fee percent="10" min_fee="4"]
Charge £8.00 for the initial item and plus 10% of the order total of which it is at least £4.00 but a max of £20.00
20 + [fee percent="10" min_fee="4" max_fee="20" ]