Unix script to find, convert and replace a value from a file

See more:

script

I have a file which contains so many values which ends with a string px. I need to find all those values, multiple them with 0.43 and again write them to the file with the substring 'pt' instead of 'px'
How do I achieve this ??

eg string from the file:

.ClassOne
{
    margin: .35em 5px;
    overflow: hidden;
    padding: 0 1.4em 0 .5em;
    position: relative;
    text-overflow: ellipsis;
    white-space: nowrap;
}
.ClassTwo
{
    border: 7px solid #FFF;
    border-top-color: #000;
    position: absolute;
    right: 6px;
    top: .95em;
}
.ClassThree
{
    bottom: 5px;
    height: 1px;
    left: 1em;
    position: absolute;
    right: 2em;
}

To find all the values that ends with px, and convert them to points by multiplying with 0.43 and write them back to the file.
Example: 5px = 2.15pt, -9px = -3.87pt

How to achieve this using a single line command line in unix?? Since I am new to shell scripting, I need some help. All I know is that it can be achieved using sed -i.