If you haven't read the complete guide here
https://igpmanager.com/forum-thread/45862 then I would suggest checking out section 3 for some very good advice on push level management.
For Boost on top of the above advice:
Try to use boost as your car is accelerating for maximum effect.
Try not to use boost when you are already at your top speed unless its absolutely necessary (for example towards the end of the long straights in Italy but before the braking zone, you do go faster, but there's not as much gain)