Seeking trial and Error help

Pn = 0.18; << Assume Value

Phil, Phiv ( is calualted with some equation using above Pn value

Fv=Pn*phiv;

Fl=Pn*phil; (Fl-Fv) < 0.00001 <<<<<<this is basically a error checking

Trail and error command are starting here

if (Fl-Fv) < 0.00001 <<<< error check so above assume pressure is right and display it if not caluclate another pressure with following relation Pnew = Pn*(phil/phiv); this new pressure will take the place of above assume pressure and again check error check if we got the our error is less then 0.00001 , stop and display the pressure, this is like trial and error and might run in a loop but i dont know how.. need help