in your css mention position right then “spacing value(50px)” then other position(center/top)
li {background: url("../img/grey-arrow-next.png") no-repeat right 50px center;
border-bottom: 1px solid #D4E8EB}
Older Browser and IE8 and old version of IE does not support this code. Latest updated browsers has no conflicts with this method and hopefully future updates will support it too.
If you are using modern browser, try background-position: calc(100% - 50px) center; as suggested in another answer too. calc has long way to go as it is logically and mathematically capable to produce much accurate result.