This is a great question, I am getting into the same boat. I just started on my pump and would like to work up to Pizza again lol.
I have been told that a safe place to start is to calc normally what you would take, and only take half pre meal, and then stretch the rest out over 3-4 hours. I think would need some adjusting but its a good start.
If you remember when you insulin pens, and ate that meal what happened? I normally found i would inject everything pre-meal, and be fairly low for about 3 hours, and then over then next 3 hours rise until i was up around 12mmol/L and have to take a correction.
Based on that I think i would take half initially, and stretch out the other half over 4 hours, and see what happens.
Let us know what you figure out! I am curious