net profit and ebitda

If a co. report gives a EBITDA figure...

Obivuiously you have to take into account tax, interest etc.

And of course tax etc. etc. varies for each co. but do any of you have a basic rule of thumb that you use to derive a Net Profit or EPS figure from an EBITDA figure??? (eg, Net Profit may be 50% less than EBITDA)

THANKS