I hope you mean "until I can do suboxone and not have precipitated withdrawals." Anyway, I suggest waiting until tomorrow night before dosing the suboxone.
.4 is a decent amount of heroin. 24 hours seems to be the standard amount of time to wait, but the longer you wait the better it will work, and the lesser the chance of you experiencing precipitated withdrawals.
It's best to wait until you are in withdrawals and need to take it. Not just feeling a little shitty, but actually feeling some real withdrawal. I always keep busy otherwise my mind will trick me into thinking I am withdrawing more than I am, and if I take it too soon it does not work that well. I suggest waiting 36 hours since IME it works a lot better if you wait that long. I have taken it after 18 hours and it didn't help and seemed to make me feel a bit worse, making me think that I cut it too close and perhaps had the suboxone knock the last few bits of heroin off of the receptors, but not enough were left for it to be severe precipitated withdrawals.
When I wait 24 hours it's like I just don't experience withdrawal after that, but it doesn't make me feel the effects of the suboxone. That's why I prefer to wait 36 hours, since not only does it take the withdrawal away, but it also lifts my mood a little. The longer you can wait the better it will work, but if you can make it that long sometimes it's better to just kick.