select random value based on condition in Jquery

please see the following html

<div class="testclass" style="top:30px;">msg1</div>
<div class="testclass" style="top:60px;">msg2</div>
<div class="testclass" style="top:80px;">msg3</div>
<div class="testclass" style="top:100px;">msg4</div>
<div class="testclass" style="top:200px;">msg5</div>

select random value that less than 623 and that random value is not in the stop style of the element ie not in 30,60,80,100,200 .But these values changes . And that random value is need to be atleast 30 more from top values . How to do this .