If using Hodgdon powder, first place to visit is the manufacturers website:
Cartridge Loads - Hodgdon Reloading Data Center - data.hodgdon.com
For the regular 243 with a 100gr bullet the load listed is from 37-40gr. With an AI the usual rule is that it may take up to 10% more powder, particularly if the bullet is seated long.
Another good source is .243 Win + .243 AI Cartridge Guide
They list ackley loads, but beware that many of the loads listed are very hot and with long seated bullets.
4350 is a good powder for the 243 and 243AI You will see that the loads on 6mmbr do not get to the 243AI+10% level, but are rather just a grain or 2 more.
I was unable to get a decent amount of 4350 which is why I am using H100V (which is relatively easy to get) and I will just have to see how consistent it is over different temperatures. I have a bunch of superformance powder which I bought by mistake and I will be trying that also. These are all relatively slow powders.
I would never start with a Min load. Rather start in the middle or even directly at 39gr. When I worked up my fireforming load, I started directly with 41gr of H100V which is the max load in the parent cartridge. Because of the expansion of the brass the pressure peaks out lower than in a regular chamber and this load has shot 1/2 moa for me.