Hey!
you win 450 40% of the time when he folds
you win 300+450 with a probability of 0.6*x, where x is your equity against his calling range.
you loose 300 with a prob of 0.6(1-x).
That assumes he made no bet before you shove (when he calls he calls the full 300 you push)
Your EV = 0.4*450 + 750*0.6*x - 300*0.6*(1-x)
put EV = 0 and rearrange:
1050*0.6*2*x = 300*0.6 - 0.4*450
Example is not very good, as we get x=0, meaning you don't need any equity. That is, your push is profitable with just foldequity alone, even if you loose the flip 100% of the time.