Get max range out of A220

What can I do if I want to maximize the range in the A220 what altitude and speed should I fly at to get max range

I can recommend SimBrief.com.

You can generate very realistic flight plans, and with the Detailed Navlog option, can see all the suggested altitudes and speeds along your entire route.

I generally just go by trial and error from trips
Like VHHH-RJTT and keep going up to something like a KJFK-EGLL if it has the fuel capacity, but sim brief is a very good tool