It does depend on how flexible you are, but the simple explanation is that having the back foot at 45 degrees makes it easier to kiss the knees and compress on the front foot.
Also, if you get used to practicing with it at 45 degrees and then it drifts more towards 90 when you're surfing, it's still in a pretty good place for you to be front on, but if you practice with it at 90 degrees, you're more likely to end up with a side on or poo man stance.
Here's a video I made for myself a while back, perhaps it will help clarify things 🙂
Commented on Anywhere in the southeast suburbs Mel...
16 Apr 23:39
Find a local skate park or pump track and follow the videos in the Intermediate course.
Then post some video and get feedback from people on here 🙂