Where should I cut the cast iron pipe. I want to replace the cast iron toilet flange and covert the iron pipe to pvc from just before the elbow going up to the toilet. My friend suggests using a soil cutter 6 inches before the elbow.

Thanks
JF

I take it that you want to cut out the cast iron pipe in the basement from a toilet on the first floor above. Not a toilet in the basement.

Show us a picture.

Cut it anywhere it's straight and smooth. You can use a sawzall or chain cutter (is that what you call a soil cutter?)
Sometimes the iron is rusted almost to the outside and shatters rather than cuts neatly, so be prepared for another cut further down the line.
If the waste line is horizontal, be sure to have support hangers especially for the iron.
Consider adding a cleanout if there isn't one nearby.